The term focal length is sometimes confused with the actual length of the lens. They are not the same thing.
Focal length is a measure of the distance between the point where the light rays converge to the digital sensor at the back of the camera where an image of the object is formed. The trick is in the use of special focusing elements in the construction of the lens that is able to bring to focus (converge) light rays that were initially traveling parallel and then project them on to the focal plane to form an image.
Shorter lenses have exceptionally powerful converging capacity compared to their telephoto brothers. These lenses are capable of converging lights rays over a very short length.
Focal length also determines the field of view. Shorter lenses have a larger field of view compared to telephoto lenses, which have a longer field of view.