The rainwater soaks into the ground or runs off the surface in rivers. Water evaporates from the ground, from the transpiration of leaves, from the breath of animals, from puddles, lakes, rivers and oceans. Notice that there is a constant reservoir of underground water. The level of this water is called the water table. Trees sink their roots down into the water table if they can, so that they have a perpetual supply of water.
People sink wells into the ground and tap the water table.
Rainfall replenishes the water in the ground. If too much water is pumped out, the water table falls, i.e., is lower down in the earth. Water tables are falling as human populations increase. This makes pumping the water out more difficult, and sometimes causes the now drier earth to settle and become more compact. This may result in permanent changes in the underground water storage capability of the area.