This multimeter add-on is called the Half Ohm. It allows you to measure small resistance values, and can be used to track down shorts on a PCB.
The board acts as a pass-through for both probes. When your meter is set to measure voltage and nothing is connected to the probes the display will read out the level of the coin cell that powers the add-on. When you are probing, the value in millivolts is actually showing the resistance in milliohms. This works for any measurement less than one Ohm. Interestingly enough, it will help you zero in on a solder bridge. By probing the two shorted tracks you can find the issue by following the falling resistance values.
[Jaanus] published several posts leading up to the final version of the board. Check out this category link for his blog if you’re interested in reading through them.