Focal length determines how much of a scene the camera sees, and it varies with the lens. A wide-angle lens can take in an entire landscape; a telephoto lens zooms in on a small subject in the distance.
Focal length is important to understand, particularly if you’re shooting with a DSLR camera. Some basic knowledge of the concept can help you choose the right lens for a particular subject and know what to expect even before you look through the viewfinder.
The scientific definition of focal length goes like this: When parallel rays of light hit a lens focused at infinity, they converge to form a focal point. The focal length of the lens is the distance from the middle of the lens to this focal point.