It's something that seems obscure and kinda confusing at first, but in practice, it's actually incredibly simple. All harmonic and melodic content is based around the tones available in a specific mode, essentially a derived scale treated in isolation, rather than set by standard diatonic function (id est, in terms of relationships to the other modes and their chords in a particular key). So, you won't encounter the omnipresent ii7-V7-IM7 (in the key of C: Dm7-G7-CM7), over which you'd play Dorian-Mixolydian-Ionian (with D-G-C as the respective roots) to maintain traditional harmonic interaction and eventually force resolution to the tonal center of the I/C.
In modal jazz, the changes aren't driven by a need to resolve tension to the I. Everything, the chords and the lines you play on top of them, all come from whatever mode you want to use, which is why you usually encounter fewer changes in most modal tunes: each new chord is the I, which is explored thoroughly to really emphasize the vibe of the chosen mode. I'm sure one of the first things in the book is "All Blues", which you could analyze as a I7-IV7-V7 (discounting the raised V7 that occurs for a couple beats in the V section), standard blues changes, but the way it's treated is as a recentering of the Mixolydian mode, with each chord, the melody and solos built directly from Mixolydian's "dominant" (Maj3+min7) tonality. G Mixolydian -> C Mixolydian -> D Mixolydian instead of just, say, the G Blues scales over all the changes.
Basically, pick one mode (viewed as a standalone scale) and build everything you play from it. Done.