Variation is one of the most important concepts in music. And no style of music does more with it than jazz. While maintaining some type of constant, usually involving melodic material and the harmonic structure, the musicians are free to add layer upon layer of ideas. In fact, one of the measures of the quality of a jazz player is how much variety they are capable of adding to a piece, during both the accompaniment and the soloing in the case of the rhythm section (i.e. bass, drums, piano and/or guitar, generally). A listener will be able to hear much more deeply into the music if he or she can pay attention to what is going on, while still being able to keep the melody and form of the song in their minds. (The musicians certainly have to do so - most improvisors like to have even the lyrics running in their minds, along with the chords and the tune.) At that point, everything that is played takes on much more meaning, because it is then heard as what it actually is: variation. So here's a great track to listen to in this way, the Standards Trio (Keith Jarrett, Gary Peacock and Jack DeJohnette) playing "Autumn Leaves": http://www.youtube.com/watch?v=io1o1Hwpo8Y. Try closing your eyes and listening to the number and depth of ideas in DeJohnette's drumming. Then try using it to do the choreography for an imaginary tap dancer. Also, notice the ending, as Peacock and DeJohnette instantly follow Jarrett to an unusual conclusion that quotes from "Speak Low".