This is a simple way to add a relay to your micro controller or CNC controller. It runs off 5V or 3.3V and most controllers can easily drive it.
The circuit is optically isolated from the relay, so your controller is protected from voltage spikes and surges.
It does this by shining light across a gap to a sensor. No electrically conductive items cross this gap.
This is a great way to add things like spindle power control to your CNC project or have an Arduino control high power devices.