Astral projection is the ability to leave your body. The way I understand it is that your body is just basically a shell that keeps you in the physical world. I've never accomplished it myself but I've heard of using it to time travel.
You have to be able to concentrate and visualize your body leaving. An open mind is necessary. Generally you will start to get cold and feel vibrations in the process. I have experienced those symptoms but I haven't got past that part.