It's when you jump out a perfectly good airplane while thousands of feet in the air, you pull a string and a big sheet opens up and catches the wind. Suddenly you go from falling to nearly flying.
When it's in the daylight and during peacetime it's great because you can see everything and the sensation is about as close to flying as you could get.
You should try it, they have lessons where they teach you how to parachute-or, sky diving as they call it now.