Plain English Meaning
A glider with its own small engine, so it can take off and climb on its own instead of being towed into the air. Once high enough, the pilot can switch the engine off and fly it like a regular glider.
Full Definition
A motor glider is a fixed-wing aircraft built primarily for unpowered (gliding) flight that also carries a small engine and propeller, allowing it to take off under its own power, climb to altitude, and sustain level flight without needing a tow plane or winch launch. Once airborne, the engine can often be shut down or retracted so the aircraft flies as a pure glider, using rising air to stay .