When shooting a wedding dress or stage performance, there will be a staff member holding a black thing, which seems to be testing. What is that? See figure

This is an instrument used to measure light intensity. The exposure meter is used to determine the appropriate exposure time in photography. When the film sensitivity and shutter speed are known, the exposure meter gives the aperture value to obtain the best exposure. Exposure meters are also used for lighting to control the brightness of lighting, or as hand-held instruments to ensure that the light intensity meets certain requirements.