As of April 2026, It is likely a typo for Windows 11 or a reference to a specific "custom" or "lite" build created by third-party enthusiasts (which are often labeled with non-standard version numbers).
I double-clicked it. It contained three items: windows 121 iso file install
If you want to perform a clean install of the latest Windows operating system, follow these steps to get the official ISO file directly from Microsoft. Windows 121 is not an official Microsoft product