This is the main switch file that controls all actions of your portfolio. No matter how you customize your portfolio, this file is the director.
For advanced users, this file offers a location where you could include content or functions you may already have on your site, into specific areas of the portfolio. I won't go into details as to how this file works. If you're editing in there, you should already know ;)
Most users of phpPortfolio will never need to play with the directory structure, re-name or customize this file. However, if you are one of those others... be sure to include the contents of this file in whatever page will be the face of your portfolio.