Expo means exponential response of servos to pilot input at the transmitter.
The control surfaces may deflect in linear proportion to the deflection of the corresponding transmitter's stick (expo mode off), or they may deflect in exponential proportion (expo mode on) (less deflection than input for small stick movements close to central position and more for extreme stick movements).
Not all radio systems have this feature.
(I don't like it myself)[sm=bananahead.gif]
"Exponential Rate: Offers servo travel that is not directly proportional to stick travel. Control response is milder below half-stick, but becomes increasing stronger as stick travel approaches 100%. Great for aerobatics and trouble situations."
http://www.futaba-rc.com/glossary.html
http://masportaviator.com/2005/09/07...ed-rc-systems/
For general RC beginners information:
http://masportaviator.com/tag/pri-fly/
This website is excellent, check it out:
http://www.gettingairborne.com/index.html