Backfill can mean two things in terms of swimming pools.
Backfill may refer to the process of removing an in-ground or semi-in-ground pool, where dirt is used to fill the void once occupied by the swimming pool after the exterior walls and bottom of a pool are removed.
Backfill can also refer to the process of adding dirt behind a new pool installation. During the installation process, the excavation contractor must remove significantly more dirt than the pool area.
This process allows the space necessary for installing plumbing and other components.
Once the installation is complete, the contractor will backfill the voids around the pool to create a seamless installation where the pool is level with the rest of the ground.