Flow in rap is more of a technical slang. Do you want to learn more? Read this article thoroughly.
When some people use flow in rap, they refer to the stream of lyrics, rhymes, rhythm, and overall composition. A good flow means rapable, rhyming well, and good to listen to.
You might hear others use it like:
“Jay-Z’s flow in his new single is whack!”
“The flow in my new rap is getting better because I got some help.”