A MAP test (Maximum Aerobic Power) also known as a Ramp Test is performed by riding at increasingly harder efforts to exhaustion. MAP tests are usually carried out in lab conditions.
There are a few different protocols, but it would typically start at about 250 watts and increase 50 watts every 2 or 3 minutes until failure.
The wattage that you complete the at last step of 2 or 3 minutes is your MAP result.