Aquifer
Aquifer is an underground layer of rock or sand that holds water people can pump up through wells.
Think of it this way
A layer of wet sand sits under a layer of dry dirt in a deep tray. The dry dirt on top does not hold water. The wet sand below is full of tiny gaps, and water fills those gaps. You can push a straw down through the dirt into the wet sand and suck water up.
Explaining aquifer by grade level
Rain soaks into the ground and fills up tiny holes in rock and sand. That stored water sits deep below your feet. People dig wells to bring this water back up. The rock and sand act like a big sponge under the dirt.