Attunement is a mechanic from tabletop 5e. It is used to make sure players can get a lot of magic items, but limits how many of them they can have active at the same time.
Most of the classes get only 3 slots to attune to, with the exception of Artificer.
The way it works is rather simple. There are magic items that you can use freely like: Weapon +1/2/3, Armour +1/2/3, Shield +1/2/3, etc.
However, majority of strong items require Attunement like: cloak of protection, cloak of displacement, boots of speed, etc.
I have seen suggestions of making items simply add +2 to atributes, but I believe that’s just a bandaid on top of actual problem, which is insane itemization.
With Attunement you have to make choice of items you want, preventing things like: Casting a spell with Phalar Aluve, switching to your main weapon in real time and then starting a fight. This would help the game balance, while still keeping itemization as important part of the game, but less gamebreaking
