There are quite a few changes, so for starters perhaps it's best to list all the things that remain: The concepts of health, AC, classes, races, feats, ability scores, skills, saves, spells, spell slots, and death saves remain mostly intact from dnd 5e. For this reason, some familiarity with the core of dnd 5e would be very helpful to help understand certain wordings or concepts.

"Subject to GM"

That said, there are some design decisions. Perhaps the largest is the change in wording of abilities, which can apply the "subject to GM" clause. This means that both the GM and the player must attempt to meet the spirit of the wording as best as possible, while allowing the GM and player to find creative ways to resolve it.

Leveling and archetypes

Another change is that of customizability and the overall way that benefits, levels, and bonuses are handed out. The maximum level is set to 12, which means that simple +1 bonuses, abilities, and customization options can attempt to make every level for every class unique. There is also a focus on making multiclassing more interesting, which is not only reflected in the more "mechanical" roles of the classes, but also in how subclasses or archetypes are designed. This means that every class has at least tiers of archetypes. This means that more varied combinations of archetypes can be weaved into the classes, as well as more varied "strengths" of archetypes. A shadow-weaver rogue might be something more powerful than a thief.

This also means that, design wise, an earlier archetype does not have to compete in power with higher archetypes. There's no reason why a scribe and an Archmage should both have "equal" capstone abilities.

Rest, long rest, and rest dice.

Another design change is a much higher focus on resources, and the importance of rests and long rests. The rest dice resource is now much more important, with each class having at least one ability or use for spending rest dice, either during adventures, combat, or during a rest. A rest dice can still be used to restore a number of health and 1 fatigue, but they often have other uses. As a consequence, short rests now require a day of rest, and long rests require a week. This is, of course, subject to GM.

"Blocking" attacks

A new combat resource has been added to the game to alter the flow of combat. All creatures now have a certain amount of "Block". This is sometimes referred to as "instances of block" or "X block." This is a way of representing a way to completely block the attack. When an attack lands(attack roll >= targets AC) the target of the attack can choose to spend 1 block and make a block check. On a success, the damage and(typically) any added effects are ignored. Consequently, all attacks have an associated block DC.

The baseline block DC of an attack is equal to 8 + proficiency + the attribute used for the attack.

The baseline amount of block gained after a rest is equal to your dexterity modifier. Armor, shields, spells, and abilities can increase this.

Some abilities, items, and effects, can grant armor "at the start of combat" which means that, if you do not lose the gained armor by the end of combat, it is not retained afterwards.