The given string will be appended to the Trac website URL when you browse the source code.
This is useful if you have multiple source code repositories under your trac website.

Example:

If you change "src/main/myfile.py" in the source code repository "myRepo", normally the resulting URL is "http://trac.mycompany.com/browser/src/main/myfile.py". Depending on your Trac configuration you need this as and URL: "http://trac.mycompany.com/browser/myrepo/src/main/myfile.py".
To get the correct URL you need to enter "myrepo" into this field.