Exposure relates to the amount of light that is used to form the image and is controlled in three ways.
The Shutter Speed, the Aperture and Sensitivity to light (ISO).
Shutter Speed is the length of time that the sensor or film is exposed to light. The shutter has a time value control running from whole seconds through to fractions of a second. The normal range on most cameras is 30 sec (very slow) to 1/8000 sec (very fast). If light conditions allow, a fast shutter can be used to freeze the subject; if you wish to convey motion in an image, then a slow shutter can be used, where any movement is rendered as a blur. Both fast and slow shutter speeds can be used creatively and is a matter of choice and what the photographer wishes to convey.
Slow shutter speeds can introduce unwanted elements, such as camera shake, so if this is not what is intended then it is advisable to use a camera support such as a tripod, monopod or beanbag, etc.
The length of shutter speed that can be used is dependent upon two things, the available light (largely beyond the photographer’s control) and the Aperture set (within the control of the photographer).
Aperture is the quantity of light allowed to pass through the lens and on to the sensor or film. If you think of the aperture as an adjustable hole in the lens that can be set to large (allowing a lot of light through) or small (allowing little light to pass through) you get the idea. Apertures are given in f numbers. Eg f 2.8, f 5.6, f 8, etc. The smaller the number the more light passes through and the larger the number the less light passes through. It does sound confusing but to attempt to understand the relationship between the f numbers imagine putting a 1 above the number, hence 1/2.8 is larger than 1/16.