What is R-SIM Unlock for iPhone?

I'm going to buy an iPhone 5 and it is R-SIM unlocked. What is that. Are there any disadvantages of this unlock method?

The R-Sim unlock method is get the phone to work on any GSM carrier that uses a sim card. It is in the sim card slot that you put your sim card in. The card works fine, however if you update your phone (and iPhones do update quite a lot, I'm a user) there is a chance the unlock will reset and not work.

However, if you do plan to use GSM carriers, I suggest buy an unlocking service on ebay which are quick and simple plus cheap and it's permanent.

Here is a trusted unlock guide I know of:
hope it helps.