The focal length of a camera lens describes the effective distance of the pinhole from the film that the lens acts like.

To put it in plain English, a lens of focal length of 50 mm (the "normal" lens for 35 mm film) will project the same size of image on the film as a pinhole located 50 mm in front of the film.

The "normal" focal length is such that the resultant image looks essentially the same as if viewed by the human eye, and is typically the size of the diagonal of the image on the film.

For example, 50 mm is "normal" for 35 mm film, 150 mm is "normal" for 4"x5" film, 300 mm is "normal" for 8"x10", etc.

A focal length higher than "normal" produces the telephoto effect; one lesser than "normal" produces wide angle images.

Suppose you build a 4x5 view camera and instead of buying an expensive lens just drill a pinhole, you can change the "focal length" by simply moving the front of the camera away from or closer to the film. You will have a perfect zoom capability. Move the pinhole 150 mm (6.8") in front of the film for a "normal" image. Move it forward (away from the film, and you can take telephoto pictures. Move it back (toward the film), and you can take wide angle shots (though these may not fully cover the film edges - the "normal" and telephoto ones, however, will).

Just remember that if you do that, your f-stop changes, hence you need to adjust the exposure time accordingly.