The value in millimetres is the focal length. A lens with focal length of 100mm will bring parallel rays of light (from an object very far from the camera) into focus 100mm behind the lens.
So clearly if the mountains come into focus 100mm behind the lens, then the length of the lens barrel must be 100mm minus the distance from the sensor to where the lens mounts. Closer objects will be brought into focus at greater distances, so to be able to focus closer the lens has to be able to extend to move further from the camera so the barrel would have to be longer.
Now that is for a single thin lens. A multi lens system a with focal length 100mm will bring the mountains into focus 100mm behind the rear principal plane. This is an imaginary plane the location of which depends on the properties of the lens elements. Telephoto designs move the principal planes forward meaning the physical length of the lens barrel can be less than the focal length.
Focal length also determines angle of view, but it is co-dependant on the size of the film or sensor. This is why it is important to understand the definition of focal length given above, as people tend to say mounting a 100mm lens on an APS-C camera body makes the lens 150mm, this is not true, a focussed image is still formed the same distance from the principal plane, it merely makes the field of view the same as a 150mm on a 35mm sensor.