The term "set piece" denotes a virtuosic scene in a story. Something designed by its creator specifically to impress his audience. Even if you've never heard the term before, you've seen it played out many times. Think of the aria from Aida, a Harry Potter Quidditch match, or the heist scene from Rififi. Some stories are built almost entirely around set pieces. Dan Brown's Angels & Demons is a good example.
The problem is, set pieces can become so grand and so focused on delivering excitement that readers lose interest and get bored—right when the author expects them to be amazed. At these moments, the set piece has taken our focus away from the characters, and inadvertently made the story about itself.