- The document discusses barriers to movement in syntax, specifically prepositional phrases and noun phrases acting as islands that prevent elements from being moved out of them. - It uses examples like "We talked about the fact that you slept for hours" to show how the syntactic structure and placement of prepositional phrases affects the meaning. - Noun phrases containing relative clauses or wh- words are also considered islands, as elements cannot be extracted out of these complex noun phrases during transformations like wh-movement. This is known as the subjacency principle.