A RAR file (short for a Roshal Archive Compressed file) is a compressed file, or data container, that holds one or more other files and folders inside of it.
Think of a RAR file like a regular folder on your computer, where there are sometimes many other files and folders inside of it for the purpose of organizing them.
However, unlike a normal folder, a RAR file needs special software (more on this below) to open and extract out the contents.