R-SIM card is a type of SIM card adapter designed to unlock GSM or CDMA smartphones, allowing them to be used with different carriers. It's often used when a device is locked to a specific network and the user wants to switch to another carrier without changing the phone itself. R-SIM cards work by intercepting the activation signals sent by the carrier, bypassing the need for an official carrier unlock. These cards usually come with instructions and may require some setup steps to function properly with the desired network.