EOS Knights is a so-called "idle" game. This means you make some actions, and then you can wait until you accumulate some stuff, then do some actions again, etc. The core element is waiting, that's why you call them "idle"
More specifically, you have up to 3 knights that are on a mission to fight as many goblins attacking a village as possible. The "story" is almost negligible, as you don't get to feel much of it. Your knights fight against goblins until they die, and then you can revive ("rebirth") them to start a new fight. Every time you rebirth, you get the loot your knights have collected from the goblins they've killed in the last battle; the more goblins they've killed, the more likely it is to get better loot. Your knights gain experience and can be leveled up, but more importantly, you can equip them with items (weapon, armor, accessory) to make them stronger - meaning they last longer in battle or can kill more goblins.