This doesn't mean rigid rhyme scheme necessarily, if you don't know what flow is, you need to listen to these raps.
Flow is felt, it is experienced, you cannot ever truly make it into a science, it has an area of it that makes no logical sense.
If you are able to 'zone out' of the song, if you don't involuntarily go 'hot damn' to the track and let it hypnotise you from the way the lyrics and track flow seamlessly, the flow is jarred in some way and insufficient to be called god tier flow.
And that's just the tip of the iceberg (of each artist as well as of rap in general).