When the small knob at the center of the drain is lifted, the drain is open and the water flows. Conversely, when the small knob is pushed down, the drain is closed and no water flows.
Bathtub drain is open
This is a photo of the small knob at the center of the drain port is lifted. In this state, water flows from the drain.
Bathtub drain is closed
In this photo, the knob at the center of the drain is down. The drain of the bathtub is closed. Water does not flow.