It means that the person saying it will be leaving you momentarily to do something else but will return to you shortly. Hope this helps!

I think this sentence means ' I will be back just in a short while' For example, you are in the restaurant. The waiter gives you the menu to order food. After you order the food, you say that you are very thirsty, and want a cup of water. The waiter may say, "I will be right back." That means he will be back very soon with the water.

I should also note that "right' in this use has nothing to do with being toward the right of something. Similarly, being "left behind" has nothing to do with being towards one's left, but does have something to do with being toward on'es back. It's strange I know :)