There are two kinds of Weapons in Tree of Savior: Sub-weapon and main-weapon(?)
Main-weapons can be used by pressing Z to deal main attacks.
Sub weapons are used in the left hand, main-weapons are used in the right hand. However, in the game you will see the contrary, because when characters are using main-weapons on the right hand, they are hardly visible on the screen so to make them more visible, ToS developers changed the side.
Sub weapons allow the player to use unique skills and some sub-weapons are not really important in the game. Sub-weapons can be used by pressing C to deal sub-attacks. Almost all Sub weapons, except for shield, can be used freely regardless of which class you use.