Plain English Meaning
On a GPS approach without vertical guidance, this is the lowest altitude you are allowed to descend to while looking for the runway. You level off here and do not go lower until you can see the runway and meet the rules to land.
Full Definition
The lowest altitude, expressed in feet above mean sea level (MSL), to which descent is authorized on the final approach segment of an RNAV (GPS) non-precision approach flown using lateral navigation only. Because there is no vertical guidance on an LNAV approach, the pilot levels off at the LNAV MDA and continues toward the missed approach point, descending no further unless the runway environment is in sight and the of 91.175 are met.