Official Aviation Definition
Groundspeed is the horizontal speed of an aircraft relative to the ground surface. It equals True Airspeed (TAS) adjusted for the vector of headwind or tailwind. Groundspeed determines actual enroute time and flight duration.
In Simple Words
How fast the shadow of the plane is moving across the ground below.
Real-World Flight Example
With a 100-knot jetstream tailwind, an airliner cruising at 460 knots true airspeed achieved a groundspeed of 560 knots.