You will need a file archiver tool. The most common and recommended tool is 7-Zip (free, open-source).
download or open the file unless you are absolutely certain of the source. derjrefkio.7z
Because this file name is not recognized as standard software, if you have encountered it online, it could be a malicious file. You will need a file archiver tool
The name appears to be either a heavily obfuscated file, a typo, or a niche, non-public archive. ⚠️ Security Warning if you have encountered it online