The Brief: A diss track is an insult in song form.

DEEP DIVE
A diss track is a song or a part of a song that directly or insults a specific person. Diss tracks have a long legacy in rap music but can be made in any genre. They are a way for rappers and musicians to drag people they have beef with.
Eminem disses Machine Gun Kelly:
Nicki Minaj disses about a dozen specific people in “Barbie Dreams,” including Drake and DJ Khaled.
Leading up to their boxing match, KSI and Logan Paul both made diss tracks devoted to insulting each other.