Quick explanation: fan fiction is a story based on a fictional/nonfiction world that deviates from the actual timeline and produces an alternate story. For example, if Naruto characters were in the modern world.
Archive of Our Own is a beta nonprofit website that distributes free fan fiction anyone can write with no advertisements at all. However, the process to write a fan fiction on said website is tedious, as the writing quality of these stories could sometimes rival
Harry Potter, bringing up competition. Archive is commonly abbreviated to ao3. This website is commonly known as the holy grail of fan fiction, where the elite stories belong due to the sheer dedication people get, the growing support (albeit anonymous), and its intricate ‘tags’ sorting system. For example, take two people, Bakugou and Midoriya. If you want to see a fan fiction with a
relationship between the two, you go to their fandom (Boku no Hero), select
Relationships, then Midoriya Izuku/Bakugou Katsuki. This fan fiction may or may not have
5000 kudos (likes).
Other fan fiction websites that are notable for being
significantly lower tier than Archive are fanfiction.net (ff.net) and Wattpad (wattpad.com).
In short, Archive of Our Own is the best fanfiction website and you cannot change my mind.