Well, without more context, it's hard to say precisely. 'PBS' might refer to the Public Broadcasting Service. So perhaps it's a story created by PBS in which a monster is the big reveal at the end. It could be an educational or entertaining story that uses the concept of a monster at the end to engage the audience, like teaching kids about facing their fears.
I'm not entirely sure as the title is a bit unclear. It could be a story about a monster at the end that is somehow related to PBS. Maybe it's a children's story on PBS where there's a build - up to a monster at the end.