In your example, the 100mm is the focal length of the lens, basically it's magnification range. The 100mm is what's known as a "prime" lens; i.e. it doesn't zoom. The kit lens, the 18-55, indicates that it's a zoom lens that's 18mm on the wide end and 55mm zoomed in.
F stop refers to the aperture of the lens at it's widest. While you do set the f/stop from the camera it's the lens that you're actually adjusting.
When you want a lens that does macro as well you also need to look at minimum focusing distance. If you have to stand 6' away your 100mm won't seem very close.
I shoot Canon but the Nikon 105 f2.8 is probably what you want. It won't autofocus on your body but for macro work that's really not a concern.