Plain English Meaning
Latches that hold the wheels rigidly in the down position so the gear can't fold up when the airplane's weight settles on it.
Full Definition
Mechanical devices on retractable landing gear that secure each gear leg in the fully extended position, preventing it from collapsing or retracting under the loads of landing, taxi, and takeoff roll.
Context Anchor
You encounter downlocks when studying retractable landing gear systems, landing gear position indications, and abnormal landing gear procedures.
Derivation
Plain compound of 'down' (the gear position) and 'lock' (a device that holds something in place). The name describes its job: it locks the gear down.