Some keyboards measure how hard you press down on the keys, and convert this to a voltage (or other control signal such as MIDI, which can then be converted into a control voltage) that you can use to add expression to a note, such as adding vibrato or opening the filter wider. Monophonic aftertouch measures one pressure value for the entire keyboard, regardless of which key(s) you are pressing; polyphonic aftertouch produces a signal for each individual key.
Important trivia: Touch plate keyboards actually measure the surface area of the skin touching them rather than pressure or force – so you can increase or decrease the aftertouch amount by rolling between the tip and length of your finger.
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