Plain English Meaning
The wheels are out and locked in place, ready for the airplane to touch down on the runway.
Full Definition
The configuration in which the airplane's wheels are extended and locked into the down-and-locked position, ready to support the airplane during landing, taxi, or takeoff. In retractable-gear airplanes, this is the position selected by moving the gear handle to the down position and confirmed by indicator lights or other visual and mechanical checks.
Context Anchor
Seen in before-landing checklists, cockpit callouts, landing-gear indications, and discussions of retractable landing gear operation.
Derivation
Gear originally meant equipment or apparatus. In , means the equipment that supports the on the , usually wheels and their supporting parts. Down describes the extended position, as to retracted or up.