Timing refers to the concept when and in which order effects will happen.
Priority is the right to play a spell or ability, or take a special action. Players cannot play spells or abilities or take special actions at a time when they do not have priority. The player with priority may put as many spells or abilities on the stack as he/she she wishes to, but before anything can resolve all players must "pass priority" without adding anything further to the stack.
From the Comprehensive Rules
(Oath of the Gatewatch
(January 16, 2016))
116. Timing and Priority
- 116.1. Unless a spell or ability is instructing a player to take an action, which player can take actions at any given time is determined by a system of priority. The player with priority may cast spells, activate abilities, and take special actions.
- 116.1a A player may cast an instant spell any time he or she has priority. A player may cast a noninstant spell during his or her main phase any time he or she has priority and the stack is empty.
- 116.1b A player may activate an activated ability any time he or she has priority.
- 116.1c A player may take some special actions any time he or she has priority. A player may take other special actions during his or her main phase any time he or she has priority and the stack is empty. See rule 115, “Special Actions.”
- 116.1d A player may activate a mana ability whenever he or she has priority, whenever he or she is casting a spell or activating an ability that requires a mana payment, or whenever a rule or effect asks for a mana payment (even in the middle of casting or resolving a spell or activating or resolving an ability).
- 116.2. Other kinds of abilities and actions are automatically generated or performed by the game rules, or are performed by players without receiving priority.
- 116.2a Triggered abilities can trigger at any time, including while a spell is being cast, an ability is being activated, or a spell or ability is resolving. (See rule 603, “Handling Triggered Abilities.”) However, nothing actually happens at the time an ability triggers. Each time a player would receive priority, each ability that has triggered but hasn’t yet been put on the stack is put on the stack. See rule 116.5.
- 116.2b Static abilities continuously affect the game. Priority doesn’t apply to them. (See rule 604, “Handling Static Abilities,” and rule 611, “Continuous Effects.”)
- 116.2c Turn-based actions happen automatically when certain steps or phases begin. They’re dealt with before a player would receive priority. See rule 116.3a. Turn-based actions also happen automatically when each step and phase ends; no player receives priority afterward. See rule 703, “Turn-Based Actions.”
- 116.2d State-based actions happen automatically when certain conditions are met. See rule 704. They’re dealt with before a player would receive priority. See rule 116.5.
- 116.2e Resolving spells and abilities may instruct players to make choices or take actions, or may allow players to activate mana abilities. Even if a player is doing so, no player has priority while a spell or ability is resolving. See rule 608, “Resolving Spells and Abilities.”
- 116.3. Which player has priority is determined by the following rules:
- 116.3a The active player receives priority at the beginning of most steps and phases, after any turn-based actions (such as drawing a card during the draw step; see rule 703) have been dealt with and abilities that trigger at the beginning of that phase or step have been put on the stack. No player receives priority during the untap step. Players usually don’t get priority during the cleanup step (see rule 514.3).
- 116.3b The active player receives priority after a spell or ability (other than a mana ability) resolves.
- 116.3c If a player has priority when he or she casts a spell, activates an ability, or takes a special action, that player receives priority afterward.
- 116.3d If a player has priority and chooses not to take any actions, that player passes. If any mana is in that player’s mana pool, he or she announces what mana is there. Then the next player in turn order receives priority.
- 116.4. If all players pass in succession (that is, if all players pass without taking any actions in between passing), the spell or ability on top of the stack resolves or, if the stack is empty, the phase or step ends.
- 116.5. Each time a player would get priority, the game first performs all applicable state-based actions as a single event (see rule 704, “State-Based Actions”), then repeats this process until no state-based actions are performed. Then triggered abilities are put on the stack (see rule 603, “Handling Triggered Abilities”). These steps repeat in order until no further state-based actions are performed and no abilities trigger. Then the player who would have received priority does so.
- 116.6. In a multiplayer game using the shared team turns option, teams rather than individual players have priority. See rule 805, “Shared Team Turns Option.”
- 116.7. If a player with priority casts a spell or activates an activated ability while another spell or ability is already on the stack, the new spell or ability has been cast or activated “in response to” the earlier spell or ability. The new spell or ability will resolve first. See rule 608, “Resolving Spells and Abilities.”