This is a board that contains a reed relay on it. You can contact the relay by approaching a magnet to it. So you can use it in encoder systems or magnet systems. The board has 3 pins on it: GND, Vcc and digital output pin. Thus you can use it with most of the microcontrollers especially with Arduino.
It is normally open, if you approach a magnet it will be closed. It can be integrated in systems like magnetic key, door alarm systems etc.
The reed relay on the board can be used both horizontal or vertical. It's output is digital. It provides logic1 (5V) output and the LED on the board goes on when a magnet approaches.
The package includes 3 jumper cables so you can connect it to your systems easily.