Instants, like sorceries, represent one-shot or short-term magical spells. They are never put onto the battlefield; instead, they take effect when their mana cost is paid and the spell resolves, and then are immediately put into the player's graveyard.
Sorceries and instants differ only in when they can be played. Sorceries can only be played during the player's main phase, and only when nothing else is on the stack. Instants, on the other hand, can be played at any time, including during other player's turns and while another spell or ability is waiting to resolve.
From the Comprehensive Rules (October 4, 2019—Throne of Eldraine)
- 304. Instants
- 304.1. A player who has priority may cast an instant card from their hand. Casting an instant as a spell uses the stack. (See rule 601, “Casting Spells.”)
- 304.2. When an instant spell resolves, the actions stated in its rules text are followed. Then it’s put into its owner’s graveyard.
- 304.3. Instant subtypes are always a single word and are listed after a long dash: “Instant — Arcane.” Each word after the dash is a separate subtype. The set of instant subtypes is the same as the set of sorcery subtypes; these subtypes are called spell types. Instants may have multiple subtypes. See rule 205.3k for the complete list of spell types.
- 304.4. Instants can’t enter the battlefield. If an instant would enter the battlefield, it remains in its previous zone instead.
- 304.5. If text states that a player may do something “any time they could cast an instant,” it means only that the player must have priority. The player doesn’t need to have an instant they could cast. Effects that would preclude that player from casting a spell or casting an instant don’t affect the player’s capability to perform that action (unless the action is actually casting a spell or casting an instant).
The subtype for instants is called spell type and shared with sorceries.
Blue and red are the two colors that most appreciate instants and reward the player for playing them.
The cards with the types mana source or interrupt became instants.