Plain English Meaning
On a non-precision approach (one without a vertical guidance beam), MDA is the floor — the lowest altitude you are allowed to descend to while looking for the runway. You stay at or above this altitude until you can see what you need to see to land safely.
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 in execution of a non-precision instrument approach for which no electronic glide slope is provided. Descent below MDA is not permitted unless the required visual references are in sight and the aircraft is in a to land using .