Weeb music is music is either popular with, or has themes surrounding, anime or otaku culture, including Touhou, Vtuber and idol music, however is not limited to these. The word is a shortening of weeaboo which is an insult used to refer to people who "think they're Japanese", which has since been adopted by Western Otaku culture and Fandom as an umbrella term for itself.

See also: weeaboo and japanese tags.
This is yet another tag that has been flooded by the tag trolling, notably of the copious quantity of Shikata Akiko side projects, however in this case the tag DOES actually fit for once.