An aquifer is an underground water supply –– one found in porous rock, sand, gravel, or the like. Your town might get its water from a lake, river, reservoir, aquifer, or some other source.
Aquifer is from the Latin aqua (meaning "water") and ferre (meaning "to bear") –– an aquifer literally bears water. Some aquifers are massive, such as the Ogallala Aquifer in the United States that stretches across parts of eight states from South Dakota to Texas. But not massive enough –– we're draining them faster than the water can be replenished.