[abulmagd]

without so many technical terms, it is increase in the air density below the wings as the ground is pressurizing the air between itself and the wings. that high pressure below the wing will in turn increase the lifting force of the wings
the ground effect also makes F1 boats sometimes to fly over the sea surface, and it was the main principal of icranoplanes which are vehicles with small wings and fly few feet above the sea, also ground effect applies on cars so F1 cars have horizontal stabilisers at the front and the tail so the cars will not fly!!