xbiking has always been a sort of loosely defined thing, but if I had to define it in a sentence I'd say it's cycling that is done primarily for adventure, with a certain disregard for the conventional cycling ethos which can kinda fall into snobbery at times.
Here's an archive.org link of a reddit thread where the creator of /r/xbiking explains it. It was by far one of my favourite subreddits, and I hope we get some similar activity here!