what is expo and do I need it on a lt-40???

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)

To answer do I need expo on an LT 40 if you don't have shaky hands then no probably not.
Expo will make your control movements less effective at the center of your stick. In other words if 10% of stick movement equals 10% of control throw. Expo will change that to less control throws. Or more control throw depending on which way you set expo. Generally expo is used by larger aircraft with big control surfaces.

Setting your transmitter up with dual control rates (high & low) is better than programming in expo for that model plane in my opinion . Set you low rate control surface movement at or a bit less than the setting in the manual go out with an experienced pilot for your first several flights and you should be fine . The experienced pilot can take it up for the first time and get the plane trimmed out and then let you fly it while it's in the air either via a buddy box or handing the transmitter back&forth and then land it for you until you get more experienced.
Expo (as already defined) is not a good idea for a trainer plane unless as stated ,you have a medical condition where you can't keep the sticks steady or have a hard time "feeling" the sticks.
If you don't have problems and decide to use it I would recommend backing it out in steps as soon as you can so you don't become dependent on it.

I would not recommend using expo on a Lt 40 assuming you are using moderate control throws even if you have 'clumsy thumbs'.

Expo is useful if you have large control surfaces and large deflections eg 3D models like Mojos.

I also have used expo on pattern models where I wanted to feed in subtle corrections to maintain the desired path of the model during things like slow rolls.

When I got my first radio that had expo I read what it did then asked my advanced instructor about it. I'll tell you what he told me and I have told my own students over time.
Go ahead and set your plane up with expo and see what it does and if you like it or not, it's not earth shaking, it just softens your controls or it can go the other way and make them more active around center. On an LT 40 it really makes no difference but it's a good plane to test it and see if you like it.
My planes are all stunt types and if I don't soften the controls you can see the planes moving from my shaky hands when I'm flying straight and level, you would never see it on a LT but you can on a pattern plane.
I start at about 35% then adjust depending on how sensitive a plane is.
I have always allowed my students to test these programs that the advanced radios have just so they know what they are and do. Flaperon is the one that always catches everyone's attention, it really got mine and I just had to give it a try. On some planes it's fun and on others it's a crash waiting to happen.
I did all my radio program testing on an Up-Roar fun fly plane over a period of time. An LT isn't the best test platform but it's a good one to see what expo does. Not needed but it will show you for future use.