What exactly means the number of mm on my lense other than focal lenght?

I am trying to get into photography and bought this fancy camera that i am just getting to know better each day. I still have trouble understanding the "photographer language". So I am wondering what exactly tells me the mm number of my camera. If I have a lens that shoots in the range between 18 - 55mm? I hope someone can help me out here a little. LOL Thanks to everyone who answers my question.