Relays are a simple way to switch on and off a bigger circuit, by sending a lower 0-5V command signal.
Any time you need to switch a circuit, your two options are, a relay, or a transistor.
Relays are an older technology, and are actually mechanical on the inside - you can hear the click - they are far slower than transistors (50ms to turn on a circuit). But they are better isolated from the rest of your circuit, and are very robust.
The maximum capacity of the switched circuit is up to 250V 10A, and the relay draws 20mA @ 5V in order to switch the circuit.
Note: this module has a jumper connecting VCC and JD-VCC, which is a setting for relay isolation. For complete optical isolation, remove the jumper, and connect a separate 5V supply to the JD-VCC pin (and GND to GND).