Plain English Meaning
Parts that hold the landing gear firmly in place once it is down, so the gear stays down and supports the airplane.
Full Definition
Mechanical devices that physically lock the landing gear in the extended (down) position so it cannot retract or collapse under the weight of the airplane during taxi, takeoff, and landing.
Context Anchor
Encountered during preflight inspection of airplanes with landing gear that can be raised and lowered.
Derivation
A straightforward compound: 'down' (the gear-down position) and 'lock' (to secure). The plural 'mechanisms' refers to the fact that each landing gear leg typically has its own locking device.