505.6 Fourth, the active player gets priority. (See rule 117, “Timing and Priority.”)
505.6a
The main phase is the only phase in which a player can normally cast artifact, creature, enchantment, planeswalker, and sorcery spells. The active player may cast these spells.
505.6b
During either main phase, the active player may play one land card from their hand if the stack is empty, if the player has priority, and if they haven’t played a land this turn (unless an effect states the player may play additional lands). This action doesn’t use the stack. Neither the land nor the action of playing the land is a spell or ability, so it can’t be countered, and players can’t respond to it with instants or activated abilities. (See rule 305, “Lands.”)
(refs: 305)