I'm confused too
Does Guru rap off key? Cause he raps exactly the same on every track. Being on key, to me, means you have to match something in the instrumental. Some note is in some key and your rap has to be in a matching or correct key.
But most rappers just speak without regards to the notes of the music.
If you mean something besides that, explain what you mean
for something to ve on key, the different notes have to fit in a scale. Tge scale is in a certain key. The notes don't necessarily have to match notes of other elements. They just have to fit the scale for it to sound harmonious. Shyt can get more complex because other parts of the world dont all use the same types of scales AND sometimes you could switch keys in the middle or something could be considered off key in a certain key but on key in another. Notes relate to eachother. And finally, shyt could be off key on purpose (think the first wave of wu solos, especially on obc4l)
Im no mysic theory student so i could be off with some of the terminology but that's the main idea. A comparison would be like the difference between matching colors on an outfit vs coordinating colors and patterns (composing on or off key) and fabrics, textures, accessories, (mixing). Your shoes are like the drum beat...
I think i need to go to sleep...