One raised note
Lydian is the major scale with its 4th degree sharpened. It is the only common scale that is brighter than major, and the brightness comes from a single move. Major's 4th is a settled note that leans down toward the 3rd; it gives the scale its sense of ground. Raise it a semitone and it becomes a tritone above the tonic, the most restless interval in music, except here it does not resolve, it shimmers. The anchor is gone and the scale lifts off.
On the piano it is the white keys from F to F: the 4th mode of the major scale, C major's notes centered on F. The lone accidental that appears when you spell it from any other root is that raised 4th.
On the staff
Why it floats
The raised 4th does two things at once. It removes the downward pull of the normal 4th, so nothing in the scale is straining back toward a resolution, and it puts a tritone, bright and tense, directly against the root. The combination is unusual: brightness without the closure major gives you. The scale sounds like it is rising and staying risen.
That is precisely the feeling film composers want for wonder, flight, and open space, which is why Lydian is everywhere in scores. The clearest pop-culture example is the Simpsons theme, whose opening melody features the raised 4th outright. Bartók wrote a study called "In Lydian Mode," its F tonic colored throughout by a prominent B-natural. Once you have heard the raised 4th named, you start noticing it in any cue that sounds magical.
Jazz hears that raised 4th as a chord tone, not a passing colour. Mark Levine plays Lydian over a major 7♯11 chord and tells students to think in the parent key rather than the chord, since C Lydian is just the notes of G major centred on C.
How Aubel handles it
Lydian unlocks seventh, the third of the modes. It pairs naturally with Mixolydian, which you met just before it: both are alterations of major by a single degree, but in opposite directions: Mixolydian darkens the 7th, Lydian brightens the 4th. The cue to listen for is that floating raised 4th, a tritone over the root that never resolves. Rounds play the scale up and down from a random root, so the thing you fix in your ear is the lift itself.