A channel relay is a large mechanical switch. This switch is toggled on or off by energizing a coil. Wiring between the I/O control and the relay output does not connect with each other, so the MCU, which controls the I/O relay, is safe. For further information about how to use a relay, read the article available at http://www.circuitstoday.com/working-of-relays.
You can buy a channel relay module from your local electronics or online stores. The following are the online stores to get this stuff: