You will need a file archiver tool. The most common and recommended tool is 7-Zip (free, open-source).

Based on a search of available information, there is no direct, recognized, or legitimate software, document, or archive named "derjrefkio.7z" known to public records.

What is it expected to be (e.g., software, document, archive)? TOSEC-PIX full sets, 2022-07-10 - Internet Archive

the file with an updated antivirus program (like Windows Defender, Malwarebytes, etc.) before attempting to open it. General Guide to .7z Files

WinRAR or PeaZip can also handle this format. Instructions: Download and install 7-Zip. Right-click the derjrefkio.7z file. Select 7-Zip from the menu. Choose "Extract Here" or "Extract to [Folder Name]" .

Because this file name is not recognized as standard software, if you have encountered it online, it could be a malicious file.

download or open the file unless you are absolutely certain of the source.