"Bars" is a quantitative and qualititative term, signifying either the measure of an individual line (that's a bar) or emphasized for how damn hot it was (that's a *bar*).
I can't speak about this perfectly but the other answers don't answer this in the way it is used in my experience. As the other guy said. A bar is a unit of measurement for music. This generally lines up with a single line in a rap. Saying that someone has more bars than someone else means that the person is able to create more unique lines of content in their raps. If the raps are of equal quality in terms of word play, rhyme etc then they have produced a 'better' rap as they have shown more content.
They mean they have more written material committed to memory.
Good question for r/rapbattles.
Cannibus has a whole book of rhymes written, used to cripple to enemies.
It technically means more material but it actually means you have more quality material. The other person could rap for 10 minutes with less "quality bars" than someone rapping for 5.
It means the rapper has more clever/witty punchlines than the other guy. Those punchlines could be straight forward disses or in the form of wordplay.
Bars are material, ammunition. If someone has more bars than you, they have more creative lines than you.
I believe in this case, the other posters talking about musical notation are correct. However, in some cases (often in rap) bars can refer to Xanax pills. Anti anxiety medication that, when taken recreationally, can really chill you out.
I had a buddy who did bars and he would try to tell me the same story five times in one sitting. I really hated him when he was on those.
In music, a bar is another term for a measure, the group of notes and lyrics over usually four beats.
The term is often used when composing music with a tight rhyming scheme for a specific purpose. A musician working for an ad agency might be asked to come up with 16 bars of upbeat music for a jingle.
With rap lyrics, particularly when they are improvised, it is a way of bragging about how prolific you are.
16 bars in a verse. Spit a hot 16 and that’s a full verse. Having bars means you can lay verses over any track. On YouTube watch Joey Badass drop bars over Future’s Mask Off.
A bar (or measure) is a group of musical beats. The most common grouping (called a time signature) is 4:4 (four notes per beat, four beats per bar/measure), but groupings can be used.
Rap, especially, uses bars to signal how fast and how long each section of rapping is. A 10,000 bar song (which is very large) would have 40,000 beats scrunched up into the length of the song, which would then tell you how fast and fluidly the rapper is performing.
If the rapper did a 5 minute song with 1,000 bars, it would generally mean he did 4,000 beats in 5 minutes, or 800 beats per minute. This would be a very fast rapper with a lot of content in that song.
You could compare this to a rapper with only 500 bars (2,000 beats) in his 5-minute song, which would be half the speed of the previous example.
If you're interested in really fast rapping, you generally want to look for a higher number of bars in a shorter amount of time.