Specifically, it stands for a file (reenc) compressed using the 7-Zip archive format (.7z). You will most often find this filename on forums or sites like Reddit or GitHub when users are sharing:
Since this filename is generic and often used in unofficial file-sharing circles, always ensure you are downloading from a trusted source and scan the file for malware before opening it. 7z file, or
Movies or shows that have been compressed into a smaller size (e.g., converting a large 4K file to a smaller 1080p version) while trying to maintain quality.
Highly compressed versions of video games intended for faster downloading.
Because .7z files are compressed archives, you need a utility like 7-Zip or WinRAR to open them and see the actual content inside.
Bundles of programs that have been processed to take up less disk space.