Harmonize
When several tracks crowd the same frequencies the mix turns muddy: the bass and a low synth fight over the low end, a pad smears the vocal's mids, and turning everything up makes it worse. Harmonize listens to your tracks, works out which one should own each part of the frequency range, and steps the others back so each track gets its own space.
Using it
Open the Mix view. Press play, so the part you want to balance is actually sounding: Harmonize listens to the live mix and can only work on what it can hear. Then click Harmonize at the top of the view.
It listens for a moment, adjusts the tracks that were getting in each other's way, and reports what it did, for example "Cleared space on 3 tracks".
What it changes
Harmonize adds a Graphic EQ to the tracks it adjusts and makes small cuts in the bands where they clashed. Never large ones.
The bass keeps the low end: whatever track lives lowest owns the lows, and other tracks piling up down there are eased back. In the mid and high bands the loudest, most present track keeps the band and quieter tracks that overlap it are nudged down. Tracks that already have their own space are left alone.
It is a starting point
Everything Harmonize does is a normal EQ move you can see and change. Open a track's Graphic EQ in its effects to look at the cuts, adjust them, or flatten a band you would rather keep. Nothing is locked.
Play the busiest section of the song before you press it, a full chorus with every part in, because it balances what is playing at that moment. If it reports no clashes, check that the mix is playing and that more than one track is sounding. Running it again after big changes is fine; it re-measures every time.
Harmonize is for clearing space between tracks. To shape one track's tone, reach for the three-band EQ instead.