When moisture in the atmosphere condenses and falls to the earth as snow or rain, we call it precipitation. Some of the rain or snow will soak into the earth. That's what becomes groundwater. Groundwater is the water located under the surface of the earth.
The underground pockets where groundwater collects are called aquifers. A lot of the water we drink and farmers use on their farm comes from aquifers. They can provide water for homes, farms or making electricity! The water that doesn't soak into the ground either runs off and joins creeks and rivers or oceans, or evaporates into the atmosphere.