Demonstratives are words used to point to specific items within a sentence. The words "this," "that," "these," and "those" are demonstratives. "This" and "these" refer to objects nearby, while "that" and "those" refer to objects farther away. For example, "this book" indicates a book close to the speaker, and "that book" indicates a book far from the speaker. Demonstratives help clarify the location or distance of an object in relation to the speaker.