A child having fun on a scooter. This photo was captured using the ‘Bulb’ mode during the day with a ND 500 filter applied.

Bulb, abbreviated B, is a shutter speed setting on an adjustable camera that allows long exposure times under the direct control of the photographer. With this setting, the shutter simply stays open as long as the shutter button remains depressed. An alternative setting common on film cameras is Time, abbreviated T, where the button is pressed once to open the shutter and again to close it. 

In photography and optics, a neutral-density filter, or ND filter, is a filter that reduces or modifies the intensity of all wavelengths, or colors, of light equally, giving no changes in hue of color rendition. It can be a colorless (clear) or grey filter. The purpose of a standard photographic neutral-density filter is to reduce the amount of light entering the lens. Doing so allows the photographer to select combinations of aperture, exposure time and sensor sensitivity that would otherwise produce overexposed pictures. This is done to achieve effects such as a shallower depth of field or motion blur of a subject in a wider range of situations and atmospheric conditions. – Wikipedia