Aftertouch could be touched on quite a bit, as it's a basic function of most boards these days. You might do a little homework on it. My quick explanation is taking a simple sound like a violin.
You push a key, and you hear the violin. Push the key a bit harder to where you've
a) already pushed the key
b) pushing a bit harder
And suddenly you hear "vibrato" or something on the violin. It's basically voice oriented meaning each sound would have it's particular aftertouch function. Many can be programed to do
several different things. It can get pretty deep once you learn how to make it work for you.
-muzakal