What is ground effect?
It's impossible to give a correct AND simple explanation I think. I would explain it like this if I was asked:

The downwash from the wings and the downward component of the wingtip vortices create a higher than normal pressure area below the wings because the air hits the ground and can't "escape", increasing lift.

Induced drag is reduced because wingtip vortices (causing drag) can't fully develop close to the ground.