Depends on which description you are referring to.
Episode show notes = blog post content. You can customize this by going to PowerPress Settings > Basic settings, check the boxes for "iTunes summary" and "iTunes subtitle". After this, you can go and create a new episode, the fields will be available to specify specific iTunes summary/subtitles respectively.
Program show notes (what i refer to the show) = PowerPress Settings > iTunes tab, edit the summary and subtitle fields here.