Axial Modes involve just two parallel surfaces – opposite walls, or the floor and ceiling. In other words, an Axial mode consist of waves resonating only along one dimension such as the length, width or height of the room. Normally the axial modes have the most strength while the oblique modes have the lowest strength. Often times the Axial modes are less of a problem than both Tangential and Oblique modes – though there is still a need for correction in most circumstances.
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.