Relay Module 10A 8 Channel
A relay is a digital switch. What it does is to allow a current to flow or not using the signal is received. It has 3 terminals, the Common, Normally Open and, Normally Close Terminal. It also has 1 coil that does the cutting or letting the current flow.
As for the terminals, the Common Terminal and the usually Close Terminal has the continuity without energizing the coil. Energizing the coil, the Common Terminal, and the Normally Open Terminal creates the continuity and cutting the connection to the Normally Close Terminal.