Yeah, Alternate Universe, which means it's like in a different world than the one we live in now.
Like a high school AU is where in the fic the characters are in high school when in real life, or in the original series, they were not. You follow me?
In the MyChem section you might find fics that portray what the boys life would've been without the band. That's an AU because the band actually happened.
It can be anything from a character being an angel to them being Don of the mafia. If it doesn't happen in real life, or the original series, then yup, it's an AU.
All very cool. c: