Steve,
Here's a couple of sites with the info.
http://www.phrannie.org/surge.html
http://www.autoformers.com/learn/how.htm
In a nut shell, it's a device that boosts power when the voltage comming to your coach is reduced for many different reasons. Reduced voltage can do a lot of damage to your Elect. equip. and shorten the life of things like your A/C etc.
Happy Camping