I can't really say I know for sure myself (so, I guess don't completely take my word for it unless I happen to be correct, hopefully), since I don't really use the term AU for my stories and prefer not to, just because, but it's like you looked up. I believe AU means Alternate Universe, in that basically it's just a way of letting peeps known what style of story you are writing, like what the setting/genre of it all is, if that makes sense? Like how when peeps write stories where the characters are wolfs and refer to it as a Wolf!AU, it shows and lets a person, aka the reader most likely, that it's a story that takes place in a build world or alternate universe where instead of things being normal, it's set in a setting where peeps are wolfs. Or say, how peeps will have Idol!AU where it basically lets the reader know it takes place within a setting or the real lifestyle one where whatever group they're using are idols in it. I think that's all it is to it.