Mechanically, once you understand moving around in your stances and how to read the UI, it's not that complicated actually.
You spend stamina by doing most things. when low, you fight worse. If you run out, you'll fall prone and need to catch your breath. That's really bad. Endurance makes your stamina last longer
You lose your balance by doing things AND by getting hit by things. If you get low, you fight worse, and If you run out, you'll fall prone and need to stand up and regain your footing. That's really bad. Agility makes you lose less balance.
grip is a mechanic... apparently you can drop your weapon if you run out of grip? I have never seen this actually happen so just ignore grip idk.
Armor only protects certain body parts, and gets damage as it protects. While that can be very complex all it really means at a surface level is that as a fight goes on too long, you will start taking more and more damage, but tougher armor slows that down.
There are other statuses that make you better or (mostly) worse at fighting. Your opponents can have them too, but if you're focusing on combat you're better off just fighting than trying to get a werewolf drunk or making the harpy horny so it lets its guard down, so you won't be inflicting them on purpose much (apart from bleeding, which is just extra damage)
Your opponents (mostly) have all the same rules you do, and most of them have similar if not the same stances. You can see their stance, fatigue, statuses, and balance, and If your perception is high enough you can see what stance they'll use next turn. Use this to decide when to attack, when to take a breather, and when to start thinking about pulling back into defense. If you and your opponent are both in blitz/berserk stance for instance, you're going to do a lot of damage but also take a lot which might not be worth it in the long term. If your enemy is defending and has lots of hp, it's probably not worth spending all your stamina on heavy attacks just yet. If your enemy is about to fall over, get to offense stance so you can drop the crushing blow on them. etc
Rarely, you'll find enemies with ranged attacks. Even more rarely, those enemies will actually back away from you to use their ranged attacks. If they get too far away from you, you won't be able to do damage with melee, so don't waste your stamina. Just use moves that say they get you closer until you're ready to hit them again.
Note that a yellow move means you'll be very low on stamina/balance after doing it, while a red move will run you out of one of the two, and you will fall prone immediately after using the attack. Either avoid red moves or make sure they finish the fight, or you're probably going to get punished for them.