Neutral wiring (usually white in color) completes the circuit of your home's electrical system by bringing electrical currents back to the power source. It keeps the power flowing at all times, and that's what smart switches need. Even when the switch is turned off, a typical smart switch needs some amount of power to stay connected to your network and know when to turn back on.  Some smart switches function without a neutral wire, but there is often extra technology (like a specific smart hub) at play to bypass the requirement, making them generally more expensive and limited in options.