A current relay is an electrically operated switch. Current flowing within the coil of the relay generates a magnetic field that pulls a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most should double throw(changeover) switch contacts.
Relays provide one circuit to change a second circuit which can be entirely separate from the first. For case, a low voltage battery circuit can utilize a relay to switch a 230V AC mains circuit. There is no electrical contact inside the relay within the two circuits, the link is magnetic and mechanical.
The coil of a relay passes a relatively large current, typically 1A – 45A for a 12V relay. Most ICs cannot give this current and a conductor is usually used to increase the small IC current to the larger value required for the relay coil.