Relays are activated with an electric trigger signal. When the relay is activated, it supplies a component or a circuit with power. This component can be pump, motors, circuits, etc. So the relay consists of two parts, the electromagnet which uses a plus and an earth/ground to move the switch and the actual switch which powers the component. Relays function in a similar fashion, however their configuration may vary.
The terminals are usually marked with a number.
A relay is more prone to failure when it is warm. To test a relay, listen to whether the switch is being activated with the circuit is turned on. If you cannot hear the ‘click’, then remove the relay and test the terminals as explained above. If the relay is verified to function correctly, check with a voltmeter, whether the wire to terminal 86 and 85 are being powered when the circuit is turned on. If these wires have voltage, check the wires to terminal 30 and 87/87a has voltage.