I am a lagomorph enthusiast. I want your bones.

"AU" stands for "Alternate Universe" or "Alternative Universe" (same difference) and is basically any thought scenario for a fandom that isn't canon and can't fit within the canon universe. If it takes place in the canon universe but something is notably different, that is typically what's known as a "Canon divergent AU," because it diverges from canon.

an AU can be absolutely anything. There's a couple of widespread pan-fandom au scenarios that often get thrown around, like coffee shop aus, genderbend aus, hanahaki aus (hanahaki is a whole thing in itself i'd recommend researching on your own), etc. One you might hear sometimes is "crossover AU" which is when you have characters from one fandom interacting with characters from another.

You can have as many aus as you want. They can be whatever you want and you can do whatever you want in them. It's a sandbox for you to play around in and explore how things would be different or how the characters would act in those circumstances or environments. Maybe they have different relationships with each other. Maybe they behave slightly differently. Or you can just say "Okay, [x] is true. How did they get here? How would things have to be different for this to occur?" which can also be fun.

If you are ever confused about why people ship something that seems completely out of the blue or doesn't make sense to you in the canon setting, there's a good chance they like it in an AU setting! Not everything everybody is interacting with is necessarily the canon! Not everybody wants things to exist in canon and just want to explore playing dolls in a different sandbox and that's okay. And their sandbox might look a lot different than yours, and that's also okay. You have the freedom to make your sandbox whatever you please. Do whatever you want forever. Get funky with it. AUs are fun.