Expo is a concept that is very difficult to put into words. It is a program that can be set up in some radios whereby the amount of travel on the selected control surface is not linear with the amount of movement given to the stick that controls that surface. To attempt to explain it, let's use the elevator for example. The first 10 percent of movement on the elevator stick will only move the elevator one percent of the way. Now add another 10 percent of movement on the control stick (you have now moved the stick 20% of its available travel) and the elevator will now have traveled 5 percent of its available travel. Add another 10 % of movement on the stick and the elevator will now have gone 10 percent of its possible travel. You have now moved the stick 30% of its available travel and the elevator has only moved 10% of its available travel. The amount of travel of the surface keeps increasing with each 10% of the stick movement until at the very last 10% of the movement of the stick, you get a lot of travel on the elevator itself, much more than you got on the first 10%of the stick movement. This can be plotted on graph paper. With zero expo programmed in the radio, you will have exactly linear movement of the control surface represented by a 45 degree straight line on the graph paper. With expo programmed into the radio, the line starts out almost horizontally, gets increasingly steeper as more travel on the stick is added, until it is almost vertical with last bit of movement of the stick. The result is an arc on the graph paper, starting out almost horizontal and progressing to almost vertical at the end of the stick movement. The end result of all of this is that you get almost no movement of the elevator with the first bit of stick movement, increasing to a lot of movement of the elevator with the last bit of stick movement. The gives the "soft around center" effect that we often speak of.
I am only familiar with Futaba radios, and Futaba has even incorporated this in their higher dollar radios. I have a 10CG radio that actually shows the line I was talking about earlier. With no expo programmed into the radio, the line is a 45 degree flat line. As you program more and more expo in to the radio, you can watch that line become more and more of an arc. starting out almost horizontally and progressing to almost vertical at the end of the stick movement. And that represents how your elevator will move on your airplane if you use expo.
I realize that I have not explained this well. To folks who don't yet visualize in their heads how expo works, this will be very confusing. Maybe somebody out there can help me by clarifying what I am trying to say but am doing a poor job of.