Focal length. You can think of it as how "zoom'ed" in you can be.
Normal vision in humans is considered to be 50-70mm. At 35mm you're talking about your entire field of vision, including peripheral. Anything more (over 70) is considered telephoto, and works like binoculars for the camera.
When you're looking at DSLR lenses, remember that unless you're using a high end Canon, or are purchasing the D3, you will need to take that focal length on the lens, and multiply it by either 1.6 (canon) or 1.5 (nikon, sony, etc) ((oh, and Olympus and Panasonic, you'd multiply by 2). So to clarify that, if you have a Nikon camera, which the 105 macro is used by, you'd be looking at a ~150mm macro lens.
I can explain to you where the focal length mm terms come from, but you probably don't care, and will likely confuse you.