To understand Astral Travel you must first understand that our world is said to exist on many planes, in many forms, in many dimensions all at the same time. The two most common forms are the easiest to understand physical and spirit. We believe both exist although they are in sharp contrast to each other. They are separate yet they coexist as one. Our spirit resides in our body yet the two are still separate! When the body dies the spirit jumps ship no longer chained to this world. Through training and meditation it is possible to seperate our spirit senses from that of our 5 physical senses! Most people describe this as a 6th sense or a psychic sense.
We hear about mediums and the like all the time. They open their spiritual eyes and ears and listen to what the other world has to tell them! If you have a séance and you talk to your dead father is it real? Yes! Is it really him? Maybe. He obviously now exists in a different form then he did but it is no less him and now through your spiritual eyes and ears you are now interacting with a different plane or existence then your used too.
Now let's take a different approach. Let's say you go into a meditative state and visualize your favorite restaurant. You see it as if your standing there. Every detail becomes vivid, every smell. You hear laughter and talk just as if you were actually there. Is what your seeing really happening there? Perhaps. Maybe not at that particular point in time. Maybe in the future maybe in the past. The place might have importance to you. Maybe you'll meet your future mate there and while your seeing yourself there a person approaches you and you strike up a convo. Will you meet this particular person in the future? Maybe. But what you have just done is Astral Projection.