Zoom lenses are the opposite of prime lenses. Zoom lenses have the ability to change the focal length. I.e., they have a variable angle of view. The advantage of zoom lenses over prime lenses is that you can stand at the exact same spot and yet change the field of view allowing you to shift between a wide angle and a narrow field of view.
There are tiny elements inside the lens barrel that allows you to achieve this, i.e., change the focal length. Between zoom lenses and their prime brethren, the former tends to be a little on the heavier side. This is because they have extra elements which assist in the zooming function.
Additionally, these lenses have an optical quality that is slightly inferior to that of their comparative prime counterparts. That said there are plenty of zoom lenses which are outstanding in handling and image quality.