Some rooms have cracked walls that, if damaged, break open to reveal a secret room. There is one secret room on every level.
Rewards include: A perk card and artwork displaying a previous game by the developer. Additional information about the game is displayed in a very short cutscene upon getting closer.
Finding these rooms in every level is highly recommended, as they add 5 more perks per game.
Such rooms can be located anywhere, even in boss rooms or starting rooms. (They do not appear in challenge rooms.) They are relatively easy to find by looking around the room for places where regular doors could've been until you find the cracked wall.
Certain cracked walls are easier to identify than others.