Green’s Dictionary of Slang

bar n.³

(UK/US black) a rap lyric.

2016		Skepta ‘Lyrics’ 🎵 Lord of the Mics, you was spitting that heat / But right now, your bars ain't on fleek.
2016		Jeezy ‘Never Settle’ 🎵 I'ma leave the bars to Kendrick and Cole I speak that real life.
2020		G. Krauze What They Was 30: It was a wrap after that [...] I couldn’t even spit the rest of my bars.

In phrases

have bar (v.)

(US) to be a talented rap lysricist.

2018

E. Acevedo Poet X 180: Nicki Minaj [...] GOT BARS . . . by which I mean to say, she is very artistically talented! She’s not just a great ‘female rapper,’ she’s a great rapper, period.