"AFR explains how concrete slab moisture barriers help control moisture under your foundation & how they are used for foundation maintenance.

Keeping the right amount of water in the soil is important for foundation maintenance.
With too little water the soils shrink and your foundation goes down.
With too much water the soil expands and pushes your foundation up.
To help control situations where too much water is entering the soil, engineers sometimes call for the construction of a moisture barrier.
The purpose of the moisture barrier is to keep ground water from moving under a foundation.
Moisture barriers are vertical barriers buried in the ground that are designed to prevent water from moving through the ground.
For example, a barrier might be glued to the side of the foundation, go down a foot, come out three feet and go down another three feet.
Moister barriers can also be made out of concrete or any other water proof material that is approved for the purpose.