According to the search results, "AU" in TikTok can have two different meanings:

1. Alternate Universe: This refers to a type of fan fiction where the author imagines a different version of a story or characters, often in a different setting or time period. On TikTok, users create videos that depict themselves or their favorite characters in alternate universes.

2. Gold: "AU" is the chemical symbol for gold, so it can also be used to refer to the precious metal. This meaning is not specific to TikTok and is used on various messaging apps.

Its important to note that the first meaning, "Alternate Universe," is the most common usage of "AU" on TikTok.