In search results it is an information-dense bible of programming knowledge.
If you click on the feed of newly asked questions, it is filled to the brim with idiots with barely any understanding of programming whatsoever.
The magic of their voting system and google's search algorithms converts the latter into the former.
In addition to that, there's a lot of peer review going on. First posts, late answers, low quality posts, suggested edits, and so on are all reviewed by the community. Once you start reviewing, you'll quickly notice lots of shoddy content.
Some programming language and software product vendors encourage quantity rather than quality in questions posted to Stack Overflow because various popularity rankings use the number of Stack Overflow questions as one criteria when calculating rank.