This little board allows using the rotary dial of an old phone as an input for modern electronics by translating the signals into serial data.
You need to find the right contacts from the rotary dial mechanism and the phone hook (if you want to use it) and connect them to this board. The board will work with 3.3V or 5V and uses very little power so it can be driven directly from a Raspberry Pi or even an ESP or similar microcontroller.