Plain English Meaning
When an airplane gets very close to the ground -- within about one wingspan -- the ground changes the airflow around the wing in a way that makes the wing more efficient. The airplane feels lighter and slipperier through the air for a short time, just during takeoff and landing.
Full Definition
A temporary increase in lift and reduction in induced drag that occurs when an airplane is flying within approximately one wingspan of the ground. The proximity of the surface alters the airflow around the wing, reducing the downward and outward flow of air behind it and weakening the wingtip vortices, which lowers induced drag and allows the wing to produce the same lift at a lower angle of attack.
Context Anchor
Encountered during takeoff just after liftoff and during landing as the airplane gets close to the runway.