You simply have to imagine that when you press a key on your MIDI keyboard without immediately releasing the key, you can further apply more "pressure" to the key you are holding and that right there is "aftertouch"
So aftertouch is part of MIDI messages that is sent to your virtual instrument or virtual synthesizer when you further hold down or continue to depress a key on your midi keyboard.
In most cases aftertouch is used to control various synthesizer parameters to create movement or interesting articulations.