To use a ‘long exposure’ means to slow the shutter speed down slower than one would normally shoot at, which allows more light to hit the sensor. There are many reasons to use long exposures – the most common of which being both to capture something in very limited light and/or to intentionally capture motion blur. In most cases, a tripod is needed in order to either capture a stationary object in very low light or to freeze some elements of a frame while capturing the movement of others while the shutter is open.
An understanding of manual photography is essential for experimenting with long exposures, as slowing your shutter down will likely require you to limit light in other ways (e.g. using a low ISO or closing up your aperture one stop at a time and checking results). There may also be some trial and error involved – you may find that your shutter is too slow for what you’re attempting to capture, or conversely, you may find that, even though you slowed it considerably, a slower shutter speed would suit your needs better. Keep in mind, slowing or speeding your shutter will affect how much light hits your sensor, so you may need to compensate for that light in other ways.
I enjoy using long exposures to capture the blur of lights. Whether it be traffic on the freeway, the trails that the stars take as they (we!) move, or my children playing with a flashlight, I find my results to be fascinating. These types of photographs satisfy my constant need to create, create, create, without feeling caught up taking the same old photograph, day after day. I know long exposure photography has helped me get through multiple 365 projects. I continually find myself drawn to the movement of lights and the glow of color that can be captured with a slow shutter.