Focal Length is something you can find in Camera RAW image files as well as raster images and other picture files that are derived or saved from a camera photo. It gives information about the lens used to take the photo in question.
The Focal length is responsible for the angle of view captured by the lens. It does not represent the actual length of the lens. Instead, the Focal Length is a calculation of the optical distance, based on where the light hits the sensor to create a crisp image.
A long or high Focal Length results in a narrow angle and high magnification of elements in the photo. Likewise, a shorter or smaller Focal Length will produce a wider angle of view with a low magnification.
Different lenses support different Focal Length. The so-called prime lenses have a completely fixed Focal Length, while so-called zoom lenses support different Focal Lengths.
The values given for this meta information is usually expressed in numbers. Sometimes, it’s just numerical, sometimes an additional, explanatory mm is given as well. In all cases, the Focal Length is always given in millimeters.