Hi Nobby,
afertouch is just another controller (like the modulation wheel) which can be applied to a sound to modify it. For example, pressing a key harder after hitting hit you can make a sound become louder, to emulate the way a trumpet sounds; or you can increase the amount of vibrato applied to a sound. Most keyboards let you program the aftertouch, its amount and the specific parameter controlled.
There is much more to say about it, and maybe you should read a keyboard's manual.
Andrea