i don't understand the exact meaning of this sentence in bold.
[The origins of jazz are as richly textured as the music itself. The term "jazz" really covers many different kinds of music.]
The clue is in the rest of the sentence. "Jazz" covers many types of music. In a similar way, its origins are many and varied. A fabric can be richly textured in that it has many different types or colours of thread in its make-up. The writer is trying to convey the idea that jazz is derived from many different areas and influences, all of which are as fascinating as the jazz itself.