3 or 4 months ago I'd never heard of this website. Now all of a sudden it's in the top results in my daily searches like 5x a day. How did it come out of nowhere like that? Am I just losing my mind?
SO has been King Of Google for much longer than 3-4 months! But yes, it did rise to the top of SERPs unusually quickly. Possible reasons:
it's got loads of real, unique, massively-cross-linked content. Search engines love this;
it makes all that information easily accessible without a ton of extraneous markup/scripting or bogus cloaking (unlike That Other Place that used to come top for technical searches);
the technical nature of the site is particularly suited to keyword searching;
the reputation system and moderation keeps the worthless clutter down (compared to the unusably noisy general-purpose Q&A sites);
there are lots of incoming links and relatively few outgoing ones that aren't
This is bit of a default answer but probably the simplest answer is to check out the FAQ. To sum it up it's a Q&A site for programmers by programmers with some extra features to get the "best" answer.