Plain English Meaning
How fast the airplane is actually moving across the land below it. If the wind is pushing you along, your groundspeed is higher than your airspeed. If the wind is pushing against you, it's lower.
Full Definition
The actual speed of an aircraft over the ground. It equals the aircraft's true airspeed adjusted for the effect of wind — increased by a tailwind component and reduced by a headwind component.
Context Anchor
Pilots see GS during flight planning, navigation, and on many cockpit displays, especially when checking time to the next airport or checkpoint.