Relays are basically electromechanical switches that can control a small circuit. The Relay is a digital normally open switch that controls a relay capable of switching much higher voltages and currents than your normal Arduino boards. When set to HIGH, the LED will light up and the relay will close allowing current to flow. The peak voltage capability is 250V at 10 amps. Please exercise great care when working with mains voltages— if in doubt contact a professional such as a licensed electrician for help.For all Grove users (especially beginners), we provide you guidance PDF documents. Please download and read through Preface - Getting StartedandIntroduction to Grove before your using of the product.