"USA" indicates the North American region, and "RF" typically stands for Region Free , meaning it has been modified to work on consoles from any territory.
The file is a compressed archive containing a digital copy (ROM) of Mario Kart 7 for the Nintendo 3DS, specifically hosted by the site Ziperto. File Breakdown & Review MK7-USA-CIA-RF-Ziperto.rar
The naming convention is standard for the scene. These files are generally reliable if downloaded from the official site, but because it is a .rar file, you will need a utility like 7-Zip or WinRAR to extract the actual .cia file inside. "USA" indicates the North American region, and "RF"
Note that downloading ROMs for games you do not own is considered copyright infringement. "USA" indicates the North American region