Tangential Modes involve two sets of parallel surfaces – all four walls, or two walls the ceiling and the floor. In other words, Tangential modes involve two dimensions, the length & width, length & height, or width & height of a room and are about half as strong as Axial Room Modes. Room modes can interfere with accurate referencing, and are best dealt with by purchasing and properly installing acoustic treatment in your room.
Generally speaking, a room mode is essentially a “bump” in a room’s frequency response that is facilitated by the room’s dimensions and the way those dimensions cause soundwaves to interact with each other.