its basically a way to visit the spirit realm without dying. its more complicated than that but that is basically what it is.
everybody has 3 bodies, the astral body, the physical body and the etheric body. each is within the others. your etheric and astral bodies are connected to your physical body through a silver cord. when you die, that silver cord is cut off and your astral bodies cannot return to the physical bodies. although in history, a few people have done that such as jesus christ (if you choose to believe that), and other people who have had near death experiences or who have died in hospitals for a few minutes but doctors revived them. I mysel was pronounced dead for 5 minuted due to a heart problem and doctors revived me. anyways, when you teach yourself to astral project, you are "loosening" your silver cord and so when you astral project, your astral body can seperate from your physical body without breaking your cord. you probably have had dreams in the past where you have seen your body sleeping and you were away from it or something. that was astral projecting. we do it when we dream but we dont know it.