Modal jazz is jazz played using musical modes rather than chord progressions.
An understanding of modal jazz requires knowledge of musical modes. In bebop as well as in hard bop, musicians used chords to provide the background for their solos. A song would start out with a theme, which would introduce the chords used for the solos. These chords would be repeated throughout the whole song, while the soloists would play their parts. By the 1950s, improvising over chords had become such a dominant part of jazz, that sidemen at recording dates were sometimes given nothing more than a list of chords to play from. Creating innovative solos became exceedingly difficult.