The focal length of the lens is the distance between the lens and the image sensor when the subject is in focus, usually stated in millimeters eg. 28 mm, 50 mm, or 100 mm). In the case of zoom lenses, both the minimum and maximum focal lengths are stated, for example 18–55 mm

How do you know when to use focal length ? 

Focal length is how far in/out the lens on the camera is zoomed. It  crops part of an image out so it isn’t in the final product/image.