The idea is to take some of the lessons from 4e's encounter pacing and bring it to 5e. Because 5e is designed for 6-7 encounters per day, but Mearls and the rest of the team dropped the ball on pacing out abilities.
So players just dump all their most powerful abilities in round 1 to try and end fights way more quickly than is intended, which trivializes the game in a lot of ways and winds up with the "I need a long rest" right before the BBEG encounter.
He talked about it on Blue Sky and you can read about it here:
D&D 5E (2024) - Mike Mearls explains why your boss monsters die too easily
Players are supposed to deal with 20 rounds of combat and typically handle 5 before taking a long rest. He does say "You can just give your bosses way more HP" but also acknowledges that it both breaks the CR structure and introduces the fatal flaw that you're -expected- to get a long rest before every boss fight so you can alpha strike and then have a reasonable duration encounter beyond the alpha.
By shifting something like, say, exertion from A5e to recover at the end of an Encounter, rather than a Short Rest, and reducing the total amount of exertion you have access to at a time, you can help to offset the nova problem. And then apply similar, but not identical, limits to each power source, you can better balance the game.
So, like, maybe Occult characters get 2 spells per encounter rather than short rest, but their spells hit a smaller area and weaken the target so other people hit them harder, and their invocations provide a boost to their at-wills so they get a similar total throughput bonus spread differently. Martial characters get 4 exertion per encounter for their combat maneuvers, and Psychics get 4 Psi Dice, but both Martials and Psychics get to spend up to 2 exertion or 2 psi dice on their encounter powers to boost their effectiveness. Arcane characters get 3 spells per encounter but their spells deal low damage in a bigger area. Divines get 2 Spells and a Channel Divinity that all apply some healing. Natures get 3 spells which always have a slight healing and control component.
And all of these things are independent of role.
Things like that.