Daybound is a keyword tied to the day/night cycle. Its reminder text says: If a player casts no spells during their own turn, it becomes night next turn. While it is day, if the active player goes a whole turn without casting a spell, the game turns to night at the start of the next turn. A permanent with Daybound then transforms into its nightbound face. The game can later return to day, flipping it back.
Daybound matters because it rewards planning around your spell count. A turn where you cast nothing can suddenly make your Daybound permanent stronger or give it a new ability. It also affects the whole table: multiple Daybound cards flip together, so one quiet turn can change every one of them at once.
