Phrygian is the third mode of the major scale and is considered to be a ‘minor’ scale due to the flat 3rd. the interval formula for phrygian is:
H-W-W-W-H-W-W

Since the III chord of the major scale is a m7 chord, phrygian will work well over this. For example in the key of C major the E phrygian mode would work well over Em7. To really bring out the sound of phrygian we need to make good use of the flat 2nd.