The given string will be removed from the path to the Trac website URL when you browse the source code.
This is useful if you have a source code repositories with subfolders for projects.

Example:

If you change "trunc/main/src/main/myfile.py" in the source code repository, normally the resulting URL is "http://trac.mycompany.com/browser/trunc/main/src/main/myfile.py". But because of your Trac configuration you need this as and URL: "http://trac.mycompany.com/browser/src/main/myfile.py".
To get the correct URL you need to enter "trunc/main" into this field, because this part will be removed the path.