It is when you leave your body in an energetic form. Children are thought to do this fairly regularly then they loose the ability once they reach a certain age where "reality" kicks in. It is also referred to interchangeable as an "Out of Body Experience" or OBE.

The general process is this... You are meditating, or sitting back in a chair to project, you body falls asleep, you feel vibrations, may or may not hear loud noises, conversations, other stuff, the vibrations may grow in intensity, your body is in SP (you can actually put your body to sleep when you want to project), and once you feel the vibrations you can state a command like, "Float now!" or "Now I leave my body!" and you will leave your physical body. Some people find a silver cord connecting their physical body to their energetic body. You may not be able to see, because you don't see with your eyes in your energetic body, so you have to state a command for that too, "Now I see!" From there you can visit the astral planes, or simply wander around the physical realm and watch your friends going about their daily activities.

For more I recommend these:
Amazon.com: Soul Traveler: A Guide to Out-of-Body Experiences and the Wonders Beyond (9780451197603): Albert Taylor: Books
- Written by a former NASA engineer, very good book on the author's experiences.

Amazon.com: Journeys Out of the Body (9780385008617): Robert Monroe: Books
- A slightly harder to read book covering the author's experiences, written by the founder of Hemi-Sync

Amazon.com: Afterlife Knowledge Guidebook: A Manual for the Art of Retrieval and Afterlife Exploration (Exploring the Afterlife) (9781571744500): Bruce Moen: Books
- Also written by a former engineer, the first book I read covering the methodology of projection where I was actually able to project.

Finally the Lucidology videos:
LucidDreamTricks - YouTube

There are worlds out there to explore - safe travels!
- DreamBliss