E-flat is the seventh note in F Mixolydian, so the chord built on it is called the ♭VII (the flat is there to distinguish E-flat from the “natural” seventh you’d get in the major scale, which here would be E natural.).
The chord structure is the old classic rock standby, I-♭VII-IV-I. The progression comes from the Mixolydian mode, which is as defining for rock as diatonic major is for classical.
