Expo, or exponential, is a setting for the responsiveness of the radio sticks. 0% exponential means that the servo responds exactly to the input of the stick as you control it. Increasing exponential means that the sensitivity around the 'home' position of the stick is limited. As you increase the movement of the stick, you get exponentially increased response from the servo. What this does is eliminate any 'jitters' you might have, i.e. if you are using elevator and your thumb gives just a little bit of aileron, exponential will make it so the receiver didn't really see any aileron.