As opposed to a fixed focal length prime lens, a zoom lens is an assembly of moveable lens elements that together allow for variable focal length (and therefore angle of view). Adjusting the focal length changes the size of the subject and the crop of the image captured without having to change the physical distance from the camera to the subject. One zoom lens can be used instead of several different fixed length lenses, meaning you don’t need to spend time changing lenses or carrying several different ones.
The convenience of a zoom lens comes at a cost, however. They are generally larger, heavier, and slower than prime lenses. In addition, they aren’t generally as sharp at any given focal length and are more likely to create aberrations and distortions.
Note that when zooming from shorter to longer focal lengths it is important to increase the shutter speed to the reciprocal of the new length, so that it is fast enough to prevent motion blur from camera shake.