The MIDI trigger lets you execute a macro when a MIDI note is received.
If the note field has the text focus, and you press the a MIDI note, then Keyboard Maestro will set the note field to the correct value for you.
The macro can execute when the MIDI note is pressed (note on), released (note off) or continuously while it is held down. This allows you to do things like have a macro execute when the key is pressed, and then a second macro execute when the key is released, for example to toggle a setting on and then off again.
You can configure the trigger to execute the macro when the note is received from a particular device, from a particular channel or from any device or channel.