+1

There are quite a few online resources from which you can start familiarizing yourself with the concept. You might try the obvious "Mil-Dot.com" for starters. A Mil-Dot Master is still a wonderful tool.

If you're using a variable-power scope, it's critical that you know whether your reticle is on the first or second focal plane. If it's on the first, the reticle will grow larger as the magnification increases. If it's on the second, the reticle will remain the same size regardless of which magnification you're using. Second Focal Plane optics require that you estimate range from the mil-dot reticle at a particular power setting. First Focal Plane optics allow you to use the mil-dot to estimate range at all magnification settings.

Really very simple once you understand the math behind it.