Priority in MTG Arena determines who has the right to cast spells and activate abilities at any given time. During each phase of a turn, the active player (the player whose turn it is) gets priority first. When a player has priority, they can cast spells, activate abilities, or pass priority to the next player. If all players pass priority in succession without doing anything, the top object on the Stack resolves or, if the Stack is empty, the phase or step ends.