If you’ve got a bathtub you probably have an overflow, that’s the round disc that prevents the water from overflowing if you fill the bathtub too high. Some of these overflows also have the control for the drain plug on a small lever.
You should check that it is securely fastened by trying to wiggle it with your hand and if there’s movement the two screws should be tightened. A loose overflow can allow water to pour down the back of the tub which could cause water damage. If its old there is a foam gasket behind it that can be replaced, an easy DIY project.