Quoth Wikipedia:

As it pertains to fixed wing aircraft, "ground effect" refers to the increased lift and decreased drag that an aircraft airfoil or wing generates when an aircraft is about one wingspan's length or less over the ground (or surface).[1] Ground effect often gives pilots and/or passengers of light aircraft the feeling that the aircraft is "floating", especially when landing.

Put differently- when the wing is within one wingspan away from the ground, you float. In ground effect you have less drag and more lift, so the airplane bleeds away energy (speed and altitude) more slowly.

Low wing aircraft experience much more ground effect than high wing aircraft.