In practical terms, focal length describes the magnifying power of a lens and is measured in millimetres.
A 300 mm lens has a very high magnification and narrow field of view, whereas a 6mm lens has a very wide field of view and may make an object appear further away than it really is. Lenses come in a wide range of focal lengths and are part of the palette of tools used by directors and photographers to create the images that make up a film. Long or telephoto lens equals high magnification, narrow field of view. Short or wide angle lens equals low magnification, wide field of view.