This is more of a "how to use a computer" type of question; does itch have a FAQ that would cover such things?
EDIT: The FAQ has a mention of .zip files under the Creating section of the FAQ but I didn't find any section for helping end-users navigate different file types.
Anyway, .rar files are a type of compressed archive, similar to .zip and .7z files. I personally suggest 7-Zip as it's cross-platform, open-source, and integrates well with menu items once installed. You can use it to open .rar archives in addition to other types.