How to Install Windows 7 From a CD or DVD Tutorial Guide Walkthrough
Most of the time, a Windows 7 clean install means to remove an existing operating system (like Windows XP, Linux, Windows 7, Windows 10, Windows 8, ... it doesn't matter) and replace it with a fresh or "clean" installation of Windows 7.
In other words, it's the "erase everything and start from scratch" process for Windows 7, a procedure referred to as a "clean install" or sometimes as a "custom install." It's the ultimate "reinstall Windows 7" process.
A clean install is often the best way to solve very serious Windows 7 problems, like a virus infection you can't get rid of completely or maybe some kind of Windows issue that you can't seem to solve with normal troubleshooting.
Performing a clean install of Windows 7 is also usually a better idea than upgrading from an older version of Windows. Since a clean install is a true start over from scratch, you don't risk inheriting any buggy situations from your previous installation.