If you've kept up with the game, or saw the recent G-Star trailer, this part of the game has probably left you a little confused. Truthfully, it took me a little while to understand how it worked myself. Combat in Peria is very unique. All combat we know of so far is done with summons. You can summon multiple types of kirana (entities that live in the world of Peria), ranging from summons that will target and fight enemies, summons that seem to support you, and summons that act as abilities/weapons.
Unlike your average MMORPG, there doesn't seem to be a skill system in place, rather you summon a skill/weapon and perform a one time action. For example, you can summon a sword to attack, after summoning you will continue attacking and can perform various combos. However at the end of your attack animation the summon will despawn, and you can choose to summon the weapon again to perform an action.
You can also summon normal kirana's to perform independent actions or even ultimate abilities. Normal kirana summons will be attached to your player character, signified by a blue line. You must be next to them for them perform actions or they start losing HP. You can set your kirana to perform actions against other monsters, or have them placed down to help you complete a puzzle.
Here's a great video going over this: