No rocket science here. Basically you are using a very big elastic band and some string to catapult your plane into the air!
50 to 100 feet of rubber tubing is anchored to the ground at one end. A tow line is connected to the free end of the tubing.
At the other end of the tow line is a ring that goes over the tow hook mounted on bottom of your glider. The tow hook is located just behind the center of gravity of the glider. The tow hook usually is adjustable and can be moved forward or ahead within a slot and tightened down. This is to allow fine tuning so that a steep enough launch angle is achieved.
To launch launch your glider using a bungee requires the pilot to start walking backwards stretching out the tubing until there is enough tension for launch.
You then throw your glider into the air and the high-start rapidly hurls it sky ward at steep angle at a steep angle. This all happens without pulling back on the stick. The only radio control input during launch is left/right as needed to keep the glider from veering off to the side.
As your plane passes over the anchor point, the tow line slips off of the tow hook and the plane is free to fly on it's own.
When learning to use this method of launch it's best to get the hang of things gradually, so the first few attempts don't try to go for height and therefore use less tension than what you would for maximum altitude. As you get more comfortable with catapulting launching you can start increasing how far back you stretch the bungee, and how much tension you use when launching. The reason for this is because the high-start launch is quick and it's very easy for a beginner to lose control and smash their glider, and possibly cause injury.
I've attached a video tutorial called R/C Glider High Start Tutorial With Mike Smith that's gets into more detail and is well worth watching.