Plain English Meaning
A backup way to get the wheels down if the regular system stops working. Different aircraft use different methods, but the goal is the same: get the gear down and locked so a normal landing is possible.
Full Definition
A backup method built into an aircraft's retractable landing gear that allows the pilot to lower the gear when the normal extension system has failed. Depending on the aircraft, this system may use a hand crank, a hand pump, gravity (free-fall), compressed gas (such as nitrogen or CO2), or a redundant hydraulic or electric circuit to extend and lock the gear in the down position.