Nightbound appears on the night-side of double-faced cards that flip between day and night. A Nightbound permanent only exists while it's night; if the game becomes day, it transforms back to its day-side (Daybound) face. These cards are typically weaker on day and stronger at night, rewarding you for controlling the day/night cycle.
The big rules nuance: day and night are a game-wide designation, not something each card tracks separately. Once the game enters night, it stays night until someone triggers the switch—it becomes day if a player casts no spells during their turn (day to night happens when a player casts two or more spells). Also, Nightbound permanents always enter as their night face if it's already night, and if neither day nor night exists when one resolves, it becomes night.
In Commander, with multiple opponents casting spells freely, locking day or night reliably is harder, so these cards flicker faces more than in 1v1—plan around that instability.