When we fly close to the ground, the airflow around the wing is restricted by the ground below. The ability for the air to flow around the wingtip and create a vortex is restricted and so the size of the vortex is reduced.
With a smaller wingtip vortex, the induced drag is decreased (sometimes as much as 40%). This effect typically occurs when an aircraft is at a height above the ground equivalent to one wingspan or less.
Since induced drag is reduced, the aircraft has improved performance in ground effect. Shortly after take off, this can give the impression that the aircraft is ready to start an impressive climb – so the pilot pitches the nose up. However on leaving ground effect, induced drag increases and speed will reduce. The aircraft starts to descend back towards the runway with possibly damaging results!
Similarly, when landing you can expect performance to increase as you enter ground effect. This causes the aircraft to float further down the runway and many pilots have rolled off the end of a short airstrip as a result!