Bulb mode, marked B on the mode dial or at the end of the shutter speed list, keeps the shutter open for as long as the shutter release is held down, rather than for a fixed duration. It exists because most cameras stop their timed shutter speed range at 30 seconds, and many scenes require far longer. The name is a holdover from early cameras fired by squeezing an air bulb on a rubber hose, which kept a pneumatic shutter open until the photographer let go.
Bulb is the gateway to true long exposure photography. Exposures of one, five, or even thirty minutes let you record star movement, smooth water to a mist, capture light trails from traffic, or paint a scene by hand during light painting. In astrophotography it allows the multi-minute frames that single tracked exposures sometimes need, and in fireworks photography it lets you hold the shutter open across several bursts.
Holding the physical button down for minutes is impractical and introduces vibration, so bulb work depends on a remote release, a cable release, or an intervalometer that can open and time the exposure for you. Many cameras also offer a related Time mode, where one press opens the shutter and a second press closes it, removing the need to hold anything at all.
Because bulb exposures are long, stability is everything. A solid tripod is mandatory, and pairing the release with mirror lockup on a DSLR, or using an electronic front-curtain shutter, removes the last sources of vibration. A self-timer alone is not enough here, since the exposure length itself is manual rather than counted down.
Metering does not work in bulb, so exposure is calculated rather than read. The math is simple doubling: every stop of extra darkness, such as adding a stronger ND filter, doubles the time the shutter must stay open. A frame that needs a quarter of a second in daylight needs roughly four minutes behind a ten-stop ND, and apps such as PhotoPills or the chart on a filter pouch do this arithmetic for you. Reviewing the histogram after each frame and adjusting the duration is the practical way to dial it in.
One caution with very long bulb exposures is heat. The longer the sensor stays active, the more thermal noise and hot pixels accumulate, which is why cameras offer long exposure noise reduction that shoots a matching dark frame and subtracts it. On cool nights this is rarely a problem, but on warm evenings or with exposures of several minutes it becomes the limiting factor on image quality.
As a starting point for night work, many photographers meter a test frame at a high ISO and wide aperture, then convert that reading to their working settings, since a correct exposure at ISO 6400 and f/2.8 scales predictably to ISO 100 and f/8 over a much longer bulb time. Keeping notes of what worked under similar conditions shortens the trial and error on future nights.