As
@Marshal Mannerheim said, softblocking on Twitter involves blocking and unblocking someone quickly to stop someone that you don't like from following you. If done quickly enough, the target will not even be aware they were ever blocked (they'd only notice if they happened to look at your feed in the small interval between the block and unblock), and you can avoid them until they realize that they're not following you anymore, at which point you might have to do it again. The idea is that you don't want them to pop up in your feed, but you don't want to let them know that you find them annoying or offensive or whatever through outright blocking them, usually to avoid a bunch of reeing about how you're so horrible. If called out, you can just say "must be a twitter glitch haha weird" before softblocking them again.
Essentially, it's the passive-aggressive way to block.