RAR files are digital archives containing multiple files. The files are compressed into the RAR archive and should be opened with a RAR handler such as WinRAR or 7-Zip. Install the RAR handler and open the RAR file with it; you will then have the option to extract the archive. This produces a folder with the files (in this case MP3s) contained inside.
If you have extracted the RAR archive of music files and the file format is not MP3, you can convert these files to MP3 using a converter. There are many programs that convert music files to the MP3 format, some of which are free-of-charge or come bundled with a trial period.
WinRAR is the most popular RAR handler, and the RAR format is derived from this program. RAR archives are used when large files require transfer over a network or need to be placed on a memory device with limited space. The RAR software is used to compress all of the selected files into an archive; this archive can be less than half the size of the original files. RAR software can create and open RAR archives. The RAR format is also used for security purposes: files can be compressed and encrypted into an archive and a password is required to access them. The desired files cannot be obtained by breaking into the archive, because the RAR handler needs to extract and decrypt them to their native format first.