The ARC is your visa into South Korea. If you temporarily leave the country on holiday, you will have to show your ARC when you check in for your flight back into South Korea, and also when you land at an airport in South Korea.
Appointment
Applying for an ARC
Renewing an ARC
Change of Address
You can change your address online. However, your ARC will not have your new address printed on it unless you go to Immigration.