Good question Acman; it's when some of the very best jazz musicians don't have anything original to play, and they just get together and play a bunch of well worn familiar jazz "cliches" and then they call it "jammin". JATP is well known for this; they assembled Charley Parker, Miles Davis, Mingus and every other Who's who in jazz, and told them what and how to play, but as long as the check was good, who cared.
When you hear this kind of music, you say to yourself, "I think I've heard that or something like it before"
Enjoy the music.