Plain English Meaning
On a non-precision instrument approach, the lowest altitude you are allowed to fly down to while looking for the runway. You stay at or above this altitude until you can clearly see the runway environment and are set up to land normally.
Full Definition
The lowest altitude, expressed in feet above mean sea level (MSL), to which descent is authorized on final approach or during circle-to-land maneuvering when executing a non-precision instrument approach. The aircraft must not descend below the MDA unless the required visual references for the intended runway are in sight and the aircraft is in a position to make a normal landing.