One lowered note
Mixolydian is the major scale with its 7th degree flattened. Everything that makes major bright is still there, the major 3rd above all, so the scale reads as major at first. What is missing is the leading tone. Major's 7th sits a half step under the octave and pulls up into it; Mixolydian's 7th drops to a whole step below, so the climb to the top goes slack instead of snapping shut. Major without the homing instinct.
On the piano it is the white keys from G to G: the 5th mode of the major scale, the same notes as C major centered on G. The single accidental that appears when you write it from any other root is that flat 7th.
On the staff
Why blues and rock live here
The flat 7th is not just a melodic choice; it is what fits the chords. Blues and rock are built on dominant 7th chords, and a dominant 7th contains a flat 7th, the B-flat in a C7. Play C major over that chord and the B-natural fights the B-flat. Play C Mixolydian and the scale and the chord agree. That is why the mode underpins so much rock, funk, country, and folk: it is the scale that sits naturally on top of the music's harmony. Jazz formalizes the same fact. Mark Levine treats Mixolydian as the chord-scale for the dominant 7th, so a G7 chord and the G Mixolydian scale are one thing seen two ways.
The harmonic tell is the I-♭VII-IV vamp: a major tonic, the major chord a whole step beneath it, and the subdominant. That flat-VII chord is built straight out of the mode's flat 7th, and once you can hear it you will catch Mixolydian across rock, funk, and folk. The fiddle tune "Old Joe Clark," which Clendinning and Marvin cite as a clear case, sits in it too.
How Aubel handles it
Mixolydian unlocks sixth, the second mode after Dorian. It is the counterpart to Dorian in the major family: where Dorian is minor with one degree brightened, Mixolydian is major with one degree darkened. Your ear has the major 3rd already locked in from the starting pool; the new cue is the flat 7th and the missing leading tone. Rounds play the scale ascending and descending from a random root, so you train the color rather than a fixed key.