Expo means that the stick movement coinsides with the servo movement on an exponential curve.

What this means in english is that without expo, you move the stick 1/4 of its travel - the servo moves 1/4 of its travel.
Half = half.
3/4 = 3/4
Full = Full

with expo - And these rates are adjustable, but as an example -

move the stick 1/4 -the servo moves 1/8.
1/2 = 5/16,
3/4 - 11/32.
Full = Full