The candle z probe is an inexpensive accessory that allows you to set the z-axis work zero for your carves with ease. It is a great way to get the most accurate work area for your projects and it works with any machine.
The gist of it is you have a conductive base and an alligator clip that’s attached to the a black and red wire. When the alligator clip (or something conductive it’s attached to) touches the base it completes a circuit and your CNC knows that. This, combined with some simple g-code, tells your machine where the actual work zero is on your material.
Inventables, for some reason, decided that the normal state of the PROBE terminal was to be “tripped” when not in use. So the hardware has a third wire (pin R) that connects the terminal to ground when it is not connected. When you plug in the clamp/banana plug portion of the hardware it disconnects this hardwire tripped state and grbl sees that the probe is plugged in for your probing routine. This also fixes any issues with the z-axis moving as soon as the bit touches the plate. You should be able to run the probing routine and the z-axis will back off after it’s done touching the touch plate.