The document discusses prepositional phrases and the use of commas with them. Prepositional phrases answer questions of when or where and are separated from the main idea with a comma if they come at the beginning of a sentence. Examples are provided of sentences with prepositional phrases that use commas correctly, including ones about passengers on a train, rooms below deck, clues in chapters, and voices in someone's head. It also explains that a comma is needed in the sentence "When Sarah Winchester passed, the house had 160 rooms" to avoid changing the meaning.