If you don't know Bill Withers, the composer of this tune, you should.
There are many ways to say, "Hello," to offer your greetings, to address someone, to extend salutations, to welcome someone. Whichever way you choose, this simple act of acknowledgement can express plenty. Within music, there are fancy ways of expressing simple ideas. Let me break it down for you:
If we follow the bassline, in the key of Fm, it goes: Db-C-Fm-Ebm-Ab-Db-C-Fm. A traditional harmonic analysis would look something like this: VI-V-i-bvii-III-VI-V-i. Although this doesn't appear to follow familiar chord progressions, the use of chord substitutions helps decorate a bassline that essentially is only playing a C-Fm, or V-i, progression. The Db can be a sub for Fm, but in this case it leans heavily on the C, which naturally falls down to the Fm, and continues to the Ebm-Ab, the ii-V of Db, to C, to Fm, and so on… If you take out the Db and the ii-V preceding it, then all you have left is C-Fm.
In addition to dressing up the bassline, chord substitutions give the music a sense of movement through the use of tension and release within the harmony. But you don't have to know the theory of chord extensions and substitutions to play effectively over an elaborate harmony. Simply recognize where tension is being applied, and where it resolves. This solo clearly stays within the Fm pentatonic scale, but creates tension with the blues scale and rhythmic syncopation and emphasis.
Sometimes, knowing when to say, "Hello," is more effective than how you say it.
Here's a YouTube video of Rebirth with Mos Def. Just the two of 'em.
Recommended reading: A People's History of the United States by Howard Zinn. Published by Harper Collins Publishing.