yes, exponential is a non-linear response to stick movement. the usual way to have it set is so that the surfaces move gently as the stick is moved off centre then more quickly as the stick approaches full travel. This makes response less 'twitchy'.

100% exponential would be where the control surface response followed a true exponential curve, like the green line on this graph:

50% exponential would be where the curve was half way between linear and exponential.....