As reservoir pressure falls, aquifer water expands and can flow in. A large, permeable aquifer holds pressure up and brings water to the producers earlier. A small or tight one barely moves, and the reservoir declines more like a closed tank.
Material balance uses the pressure and the produced volumes to infer how strong that support is. The aquifer is not the same body of water as the connate water in the oil column. One sits beside or below the accumulation and can encroach. The other was already in the hydrocarbon pores.