In that block two LEDs (Light Emitting Diode) are housed. In addition, a series resistor of each 2.2kOhm is used to protect the diodes against excessive current flow. They are optimized for 2mA current. The two resistors are connected to ground. So you can connect the device directly to a pair of outputs from the Arduino Nano Brick. In addition, the two signals at the other end are passed, so that there can be to carry out further controls.