What does percent systemroot percent represent?

Answer 1

Percent signs surrounding a word represent an "environment variable". There are many command-line based applications and scripts that need to account for differences in file locations, system architecture, etc. SYSTEMROOT is such a variable, and is automatically maintained by Windows. It is commonly used in the Windows Registry to point to the correct location of items stored in the directory where Windows is stored, which defaults to C:\Windows\ (the system's root directory).