The phrase what's happ'nin is a slang contraction of what's happening, long used in everyday American speech as a laid back way to say hello and ask how things are going. Online, the short recorded version became a reusable reaction clip because the greeting works on its own without any context. Creators lift it for entrances, comeback posts and group chat moments where a normal hello would feel flat. Its appeal is timing and attitude rather than a specific scene, which is why it keeps circulating as a general purpose opener.