Useful to monitor ex. state of microcontroller pin. The current is very small because LED is driven by the 2N2222A transistor. This is small PCB for only one output. Angled pin header is there to carry the board and to provide Vcc and GND. Plug it into the power rail of the proto (bread) borad taking care which pins go to Vcc and which to GND. Two lower pins are the GND pins and two upper pins are: left one (looking from the bottom layer) is Vcc and the right one is IN or input.