Similar to the Mandriva Penguin Liberation Front, the Ubuntu PLFis a team that builds litigious packages (patents covered or copyrighted stuff).
From the founding meeting, the summary is found here.:
The goal of the PLF Ubuntu repository is making packages for litigious software who cannot be included in the officials repositories because they are patented or copyrighted stuff (ex: w32codecs, RealPlayer, JRE Sun, Freenet, …).
And you will learn a bit more about the entries for your sources.list at the PLF wiki pages