So, because a lot of people, when they are writing rap songs, they may look up, or study R&B. If you look at rappers, like Ja Rule or 50 Cent, or even Drake, you will find that a lot more rappers are structuring their songs very much like R and B songs. And most R&B or pop songs, there's a bridge.
The bridge is where the music changes or some of the music comes out, maybe you just have the drums or you have all of the instruments come in at the same time. And this section in the song it's distinctly different than the rest of the song. It's the high point of the song; it's the bridge that begins to close out the song.
In a rap song, the way you create a bridge is something that is really more about repetitive. Something that is memorable. You can say something like "Throw your hands in the air and wave like you just don't care. And if you love hip-hop say yeah."
So, by saying something like that, you create something that's memorable. It's short, it's sweet, it's usually about four to eight bars, according to how you want to structure it. But, during those four to eight bars, you're just saying something short, and sweet and memorable. The more simple, the better. The more repetitive the better. Ad that's how you create a bridge for a rap song.