Plain English Meaning
The wheels can be pulled up into the airplane after takeoff and lowered again for landing, instead of staying out in the wind the whole flight.
Full Definition
In aviation, describing a landing gear system that can be mechanically drawn up into the airframe (fuselage or wings) after takeoff and extended back down before landing, as opposed to fixed gear that remains in the slipstream at all times.
Context Anchor
Seen in takeoff, initial climb, landing gear, and discussions.