Air passing through the rotor disk from above is accelerated downwards towards the ground below. Because the ground to some extent prevents the air from getting out of the way quickly, the air can't accelerate as rapidly as it does when the helicopter hovers at a higher altitude. This reduction in induced flow velocity doesn't reduce the angle of attack of the rotor blades as much as it does when hovering out of ground effect.
This why a helicopter can fly higher than it can take off from. I said nothing about hover, at altitude it will hover close to the ground, but will not be able to gain enough altitude to gain foward flight. But the same helicoptor would be able to maintain foward flight at that altitude, but may crash if it tries to hover.