The water table is the area between the soil and the groundwater underneath it. Above the water table is soil, and this is known as the unsaturated zone.
In this area, air from the atmosphere fills the space in between soil particles.
Below this area is groundwater, which is the result of precipitation in the area that has seeped below the surface layer of the earth.
Changes in precipitation and atmospheric conditions cause the water table to rise or fall. The water table varies by location, and it can also vary significantly within the same area, given changes in the topography of the environment.
When the water level rises, groundwater saturates the soil and reaches the surface.
In pool terms, the water table is important because if it rises too high, it can introduce extreme pressure against your inground pool and may even cause it to rise from the ground.