To cut a long story short, the concept, named after a popular poem of the same name which codified many of its aspects, reads as "It is our duty to control the affairs of the foreign savages both to ensure we ourselves never become imperilled by them since they are disgusting barbarians, and to also protect them and treat them as blameless victims requiring our eternal protection and support because they are simultaniously brainless and meek children who could never do any real harm" and was a popular excuse for the conquest for shits-n-giggles-n-shekels mentality of european and american nations.
This seemingly contradictory "darkies/kebabs/whatever are simultaneously evil barbarians and also blameless innocents needing our help" ideal is best summed up by how the White Man's Burden describes them.
Half Devil, and Half Child.
While first named in the eponymous Rudyard Kipling poem (he's the nigger who wrote jungle book) the concept existed a longtime beforehand in various forms, with the concept existing as a "we have to convert the heathen natives to the one true church for our benefit and their own good!" pretext for conquest by Spain in the 1500s and earlier than that with the Christian Kingdoms in europe expanding eastwards and northwards to assimilate slavs and germanics in the Dark Ages, and before Christianity it was the standard excuse of empires from Rome to Persia that "we have to spread our civilisation and uplift those bereft of it". It was also a concept that was independently dreamed up accross the world, with China's "mandate of heaven" schtick being almost identical with the whole "foreign barbarians = bad and smelly. we must control them to protect ourselves and for their own good and uplifting" aspect of it.
However it was overwhelmingly popular in academic and intellectual circles accross europe and america in the 19th and early 20th centuries, both among progressives and among conservatives and reactionaries, with the dual concepts of savage = BAD: we need to crush them under heel! and savage = GOOD! these noble childlike heathens need our protection and guiding hand! being embraced accross the board.
This all changed with the World Wars.
After WW1, the old empires (and basically-empires like what the US had going in the americas) started coming under heightened criticism and even being actively resisted by both inhabitants of the colonies (i.e. India and French Africa), the Dominions (i.e. Canada and Australia) and even the Home Nations since the trauma of the worst war in history (at the time) being the direct result of Imperial ambitions made people all over the world start to think "hey...maybe this whole empire shit is not a good idea".
This was doubly true in Academia and Intellectual circles, since unlike today they tended to be a bit more willing to sign up and fight for their stated beliefs (not because they were better or anything, just because it would have been social suicide to not do this) and thus a split formed between the "reform the empires away" crowd and the "empires are awesome and we need to double down on this!" crowd, which slowly settled on the left and right side of the political spectrum respectively.
Exacerbating this was the whole communism thing which sought to wipe away the empires alltogether (and replace them with a communist "totally not an empire guiz! plz ignore goolags" empire) and which radicalized many on the left who were anti empire, and in turn made many on the pro empire right wing side go radical themselves. However, in both camps the concept of "White Man's Burden" was still going very much strong.
In the right, the "half devil" side of things was adopted, and the "darkies are inferior and dangerous if left unchecked" aspect would often grow radicalised to the point of "GAS ANY NIGGER WHO SO MUCH AS LOOKS AT US! TAKE MORE SHIT FOR THE GLORY OF THE EMPIRE!" while ignoring the "but also do nice shit for them because they dont know any better" thing. In the left the "half child" would become the main theme, as fears of the "alien" as a threat pretty much vanished in the radical fringe and instead grew an insanely chauvinistic "IT IS OUR DUTY TO SPEAK FOR THESE SILLY NIGGERS AND ACT IN THEIR BEST INTERESTS! FOR THEY ARE NOBLE SAVAGES WHOSE NATIVE WAYS ARE SUPERIOR TO ANYTHING WE CAN DO....AND THUS WE CANNOT ALLOW OUR FILTHY VALUES TO TAINT THEM!" dogma.
Both left on their own to flail at eachother at society's leisure would have probably resulted in both teams of radicals eventually quieting down and shit stopping being so autistic on all sides. However then WW2 happened, and resulted in the far right shit becoming absolute anathema in the public eye in the west with the actions of Nazi Germany and the crumbling of European empires, while also vindicating the far left shit in the eyes of many since the West had spent years spinning the Soviets as brothers and friends, and making the public think it was no threat....right up until the cold war really sunk in.
However in Intellectual circles/Academia this effect was most pronounced. The hard/far left managed to enact a full on takeover since now their most vocal hard/far right enemies had been humiliated and shamed in the most public and extreme way imaginable. The main effect of this is that instead of both far left and far right shit being subject for attack and deconstruction on an equal playing field, Far right shit was near exclusively made a target, followed by hard right shit, then sane right wing shit, then centrist shit, and these days, sane left wing shit.
Thus the far right "half devil" version of "white man's burden" and associated racism, xenophobia, and overt exploitation, disenfranchisement, and oppression (i.e. BACK OF THE BUS NIGGERS! ALSO YOU GET FUCK ALL PAY AND IF YOU PROTEST WE LYNCH YOU!") became the subject of fanatical hatred and attack, while the far left "half child" version of the concept and associated chauvinism, cultural relativism, condescension, and covert exploitation, disenfranchisement, and oppression (i.e. refusing to criticise much less act on shit that directly hurts and oppresses people solely because other perceived victim demographics are responsible) was not only immune from criticism, but it was celebrated as the ultimate moral good
This is the basis for a lot of autistic and cancerous shit over the decades, from the modern "MUH DIVERSITY! MUH IDENTITY! REEEEEEEEEEE!" idiocy to the New age "brown people are so spiritual and enlightened and they know the true mystic secrets of the universe!" cringe OF THE 80s, to the explicit support soviet imperialism got from every "anti imperialist" group on the planet, even when the KGB was not secretly funding them, to the fawning over any dictator who says "IMMA VICTIM OF WESTERN IMPERIALISM!" even when said dictators are genocidal far right/religious fundamentalist psychopaths.
All because the left's academic and intellectual heartland was (and is) still desperately holding on to the concept of White Man's Burden, and the idea that morally correct white people have the sacred duty of protecting all the silly darkies and degenerates both at home and abroad, for their sacred native ways are soooo much better than those of the materialistic west, and whom are utterly incapable of any wrongdoing since they are but children, with everything they suffer being the direct fault of the disgusting and exploitative west which intentionally forces them to do all the things that society unjustly despises them for.
