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.
aviation.stackexchange.com/users/4/Philippe Leybaert
For a simple and correct answer you need first to understand induced drag.
Please read this answer if you feel unsure.
Short version: The wing creates lift by deflecting air downwards.
Induced drag is the consequence of this deflecting, because the resulting force of this deflection process is tilted backwards by half the deflection angle.
Due to the backward tilt, the force has a horizontal component, pointing backwards.
This is induced drag.
The ground effect will prevent this flow field from developing fully, because air cannot flow into the ground.
Consequently, the deflection angle is lower, and also its horizontal component, i.e. drag.
The same happens ahead of the wing, where the upward motion of air ahead of the stagnation point is also restricted.
Another consequence of this is a reduced lift curve slope in ground effect.
Lift is higher relative to induced drag compared to the free-flow case because the wing somewhat blocks the outflow of air at the trailing edge, and this results in a higher pressure on the bottom of the wing compared to the free-flow case.
This "cushion-effect" explanation is actually quite correct.
The pressure which lifts the airplane is not only created by accelerating air downwards, but also by ram pressure.
This effect disappears once the wing is far enough from the ground.
aviation.stackexchange.com/users/1961/Peter Kämpf
The ground acts as an aerodynamic mirror.
When the wing approaches the surface it is as if an inverted wing comes from below.
Their high pressure areas amplify each other, increasing the efficiency and thereby reducing the drag.
aviation.stackexchange.com/users/19/DeltaLima
Here's my simplification, it's not the full story, but it covers the essentials.
Lift:When you are in free space, the high pressure below your wing dissipates into the surrounding air.
When you're in ground effect, the high pressure below the wing encounters an incompressible solid, and therefore cannot dissipate as quick, causing higher pressure below the wing, and therefore more lift.
Drag:When you're in free space, the wake (turbulent, low pressure) behind your aircraft dissipates unimpeded until it becomes uniform with the surrounding atmosphere.
When you're in ground effect, the air near the ground absorbs this dissipation quicker because it has to shear against the ground (more friction that air in free space).
Additionally, wing-tip vortices encounter the ground and dissipate much quicker than in free space.
I suspect the reason GE is tied to wing length is because the longer your wing is, the stronger your wing tip vortices, and the further air has to travel laterally to exit the high pressure zone under the wing.
aviation.stackexchange.com/users/206/MishaP
Simple explanation:
When out of ground effect, air on the bottom of the wing has plenty of room to move around.
When in ground effect, the air on the bottom of the wing can't move around nearly as well and therefore pushes the plane upward to try and make more room.
aviation.stackexchange.com/users/178/Andy Levesque
in normal flight high above the ground, wings create lift by deflecting an air stream downwards.
This is not possible close to the ground: air does not travel through the ground surface.
Picture
the downward air stream being bounced back and increasing pressure underneath the wing.
This happens partially at the wing, not only behind it, because at sub-sonic speeds the air is being split in front of the wing.
So in ground effect, lift is created by pushing air downwards and by increased static pressure underneath the wing, like a hovercraft.
And the increased pressure does not come with an induced drag penalty.
And that is the main difference: total drag is lower in ground effect than in normal flight.
aviation.stackexchange.com/users/21091/Koyovis
The wing pushes air down.
In flight, that air pushes on other air that moves.
In ground effect, that air pushes on the ground that doesnt move.
Pushing things that move is more difficult than pushing things that dont move.
Like walking on sand vs walking on pavement.
aviation.stackexchange.com/users/11969/ericnutsch
What happens behind the trailing edge is surely irrelevant.
The wing has an angle of incidence to the ground so there is a wedge effect under it, presumably increasing pressure under the forward part of the chord and less so under the after part due to venturi effects.
Looks like a cushion effect to me and the airflow over the wing is deflected, and pressure reduced, as it would be at altitude.
aviation.stackexchange.com/users/27786/Pod
We've all heard the "acts like a cushion of air" explanation tossed casually around by CFIs.  There's also plenty of books and reference materials that give a detailed, accurate, and complete explanation of ground effect, and which are incomprehensible to most student pilots.  What's the simplest way to accurately explain ground effect?
What is ground effect?
aviation.stackexchange.com
<aerodynamics><ground-effect>
aviation.stackexchange.com/questions/311
aviation.stackexchange.com/users/167/Steve V.
 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. It is a complicated topic, however here is a simple answer, when close to the ground the air below the plane has little space to move, pushing the plane upwards.
Users explain the ground effect.