What is the meaning of Stop_up?

To fill a hole or cavity, or block (an opening or passage), as with a plug.

To permanently close or block (a road or path); to legally extinguish a right of way.

To increase the aperture of a photographic lens, moving from an f/stop represented by a higher number to an f/stop represented by a lower number and causing more light to pass into the camera.

Source: wiktionary.org