Both sand and DE filters use media (sand, or DE - diatomaceous earth, which is literally algae skeletons => Wiki) to filter the pool water.

In sand filters, the sand is kept in the pool by strainers, now usually plastic, often called "laterals". These are usually arranged at the bottom of the filter like spokes of a wagon wheel. If these get cracked, sand can leak back into the pool.

In DE filters, the DE powder is retained on fabric very much like Dacron sail cloth, which is supported on thick plastic grids. Again, if the fabric membranes get punctures, tears or failed seams, DE powder can leak back into the pool.

In your case, he may be saying "sand" but is meaning "DE".