What does it mean?

[mugmaface]

If someone says "IT is a nice day" wtf does "it" mean? We could stand in a building and point to the outside world and say "It is a nice day out there" but we would get no closer to understanding what "it" is.
It is going to rain soon. Q: What is going to rain soon. What does "IT" mean? It's happening now, it will soon be 9 o clock. What will rain soon, what will soon be 9 o clock.?
I just dont understand the word "It"

 

[KittyGremlin]

It means war. Next time you see him, he will be out to kill you. Be sure to have your rifle close and shoot first to catch him off guard.