A throttle lock is a device used to prevent the throttle from closing on a motorcycle. This allows the rider to remove his or her hand from the right handlebar twistgrip, which alleviates fatigue on long rides.
Motorcycles are controlled in cruise operation primarily by the right hand, which controls the front brake via a hydraulic lever (the main braking force is applied here) and the engine's throttle via a spring-loaded rotating hand grip. When the hand controls are released, they return to their resting position.
Some motorcycles, primarily those designed for long distance riding, are equipped with a device that locks the twist grip in place. This allows the rider to rest their hand and arm muscles briefly without breaking their ride.
The Honda Goldwing, unsurprisingly, goes one step further and has a full cruise control system that allows a preset speed to be maintained regardless of terrain.