It means you are getting ready to be a woman - on the outside. You are still a girl on the inside. Take time to talk to your mother or other competent adult about your maturing. Do not take advice from your 13 year old friends - they don't know as much as you do at this age! As you mature you will start feeling a lot of things - your body will change into that of a woman, your voice will change, your hair will change. You will at some point start to menstruate, and you will be physically able to have babies. But, that does not mean you should start having sex!
Save your sex for when you are in a committed long term relationship. Marriage is best, in my opinion. Boys will try to get you to do all kinds of things to have sex with them. Don't. Save your sex for when you KNOW you are going to be in it forever! Don't you deserve the pretty dress, the nice wedding, and all that goes with it? Don't start your life with a baby at age 14!
As you mature, just remember this - it is your body, you take care of it. It is the only body you will ever have! Don't smoke, don't be promiscuous, eat right, and get plenty of rest. That way you will have the best chance of having a wonderful life!