The exposure time is the time during which the camera shutter is open. In this time the light-sensitive medium (sensor, film) is exposed. In photography it is given in seconds, e.g. 1/125 or 2s.
Together with the aperture and the light sensitivity (ISO), the exposure time determines the exposure of the photo. Both – exposure time and aperture – are in a direct relationship to each other. For example, if I open the aperture further, I have to shorten the exposure time in return to achieve the correct exposure.
A long exposure time makes movements appear blurred, e.g. to depict water flowing. A short exposure time freezes movement and is often used in sports and animal photography.
With long exposure times there is a risk of camera shake, so the use of a tripod is recommended.