Expo - It changes the ratio of the input in the middle part of your stick movements, and how it effects the initial servo movement. If you imaging your stick input to servo movement as a graph, without expo it is a straight 45 degree line, expo changes that line to a curve, so that while you overall end movement of your servo does not change, the mid stick's bit becomes (normally) less responsive in the initial part of your stick movement, and ramps up at the end of your stick movement.
3D guys normally use a LOT of expo with their huge control surfaces and large throw angles - as without it, any slight movement of the sticks would put them in a tree. The expo makes slight control changes at the stick possible with such responsive aircraft.
To answer you question on the LT-40, odds on it would be a good thing as it helps with 'messy' fingers that new guys normally have (precision comes with practice and time), but don't exceed 20-30% to start with, even better, find a guy at a club who is willing to help and who looks to know what they are doing, and discuss it with him/her, even get them to test fly your plane. Over time you may start to find that initial setting a little numb, then you'll just start to lessen the amount of expo you need (as your fingers get more precise)
Hope that helps (much easier to show the effects of expo than to explain it)