Todd's link to the pages from the manual are helpful, sort of, but Particle Playground is among the more weirdly complex and alien systems in the After Effects family. You are not alone in your "Huh?" reaction. I've been trying to figure out PP since it originally showed up (in what, AE3?) and I'm no AE slouch. I've enjoyed and completed many of the PP tutorials but I have never successfully applied PP in an active project.