‌A basic relay that can toggle the power or a connection to another device. The central port is ground. Besides it you will find a "normally open" (NO) and a "normally closed" (NC) connector.

If you splice the middle (ground) and NO connector into a wire, the wire will not be able to provide power or signal unless the relay is powered.
If you splice the middle (ground) and NC connector into a wire, it will be able to conduct power/signal  by default, and will cease to do this when the relay is powered on.