Plain English Meaning
The fastest you are allowed to fly while the gear is actually moving up or down. Above this speed, the moving parts and doors can be damaged by the airflow.
Full Definition
VLO is the maximum airspeed at which the landing gear may be safely extended or retracted. Operating the gear above this speed can damage the gear doors, mechanism, or structure due to aerodynamic loads during the transit between up and down positions.
Context Anchor
Seen in aircraft performance speed lists, the aircraft flight manual or operating handbook, cockpit placards, and during takeoff or approach when the pilot is moving the landing gear.