Originally Posted by madwi

The relay here is like the power button. When it gets a signal from the computer, it closes the circuit and turns on the outlet. That powers up the device thats plugged into it.
So say the humidity gets below the low threshold of 70%, the sensor inside the tank sends a signal to the computer. The computer sends a small electrical signal to the relay which then closes the circuit letting the electricity flow turning on the fogger which raises the humidity in the tank. It runs for a set amount of time and then the computer turns off the relay, opening the circuit, turning off the fogger.