Plain English Meaning
An imaginary flat surface set at the average level of the sea. When an altitude is given in feet MSL, it means that many feet above that surface, not above the ground you happen to be flying over.
Full Definition
A standardized vertical reference equal to the average height of the ocean's surface, calculated from long-term tide measurements and used as the zero point for measuring altitude. Altitudes given in feet MSL are heights above this fixed reference, regardless of the terrain directly below the aircraft.
Context Anchor
You will see MSL in airport elevations, chart altitudes, weather information, and before-takeoff checks when comparing the altimeter indication with the published airport elevation.