A relay is used like a switch. You can turn off/on an electrical appliance (bulb, tubelight, fan, heater, etc.) through the relay, based on a condition. You will be specifying this condition in the code.
Find this relay screw terminal in your Kit.
Look at the end of this terminal, you will find three screw tumblers like this.
Use the electrical tester to loosen the screws on the terminal.
This is how the screw tumblers will look like when they are loosened.
Cut and strip a short length of one of the wires to your electrical device (light, fan, etc.). Plug the two ends into the terminal as shown.
Use the electrical tester and tighten the screws to secure the wires in place.
This is how the relay screw terminal will look like once the wires are secure.