This is an advanced trigger — generally you should use a Hot Key Trigger if possible.
Keyboard Maestro can trigger a macro when any device key is pressed—this includes modifier keys, mouse buttons, programable keyboards like P.I. Engineering’s X-Key, and even the brightness buttons on USB connected monitors.
The macro can be executed when the key is pressed, released or repeat continuously while it is held down. The trigger can also optionally be restricted to when certain modifier keys are pressed.
For example, a macro could fire every five seconds while Control-Mouse Button 3 is held down.
Unlike hot key triggers, the pressed key is not removed or affected in any way. This trigger watches input devices at a low level, but it does not affect them, so any key presses continue to have their normal operation as well as triggering the macro. This is fine for modifiers, unused mouse buttons, programable keyboards and other unused buttons, but would likely be problematic for normal keystrokes which will continue to have some other, probably unwanted, affect.