Aftertouch is a midi message representing the pressure you apply to a key (or pad) when the key has been held down or sustained. It aims to replicate the actual behaviour and sound output of a piano when a midi keyboard is played. There are two types of aftertouch: channel aftertouch and polyphonic aftertouch. Channel aftertouch is when aftertouch messages are averaged across all keys being played. Polyphonic aftertouch is when aftertouch messages are specific to each key being played, so you’re likely to have different messages being communicated for each key.