Groundwater
Groundwater is water that soaks into the ground and fills the tiny gaps between rocks and soil.
Think of it this way
When you pour water over a bowl of marbles, the water seeps down and fills the gaps between the marbles. The marbles stay in place, but water hides in all the tiny spaces around them. Rock and soil underground work the same way. Water fills the gaps between grains, forming a hidden layer of water deep below the surface.
Explaining groundwater by grade level
When you pour water on a pile of dirt, some of it sinks down and hides below. The water fills up small holes between bits of soil. Different soils hold different amounts. Sandy dirt lets water pass fast, but thick clay holds it back.