Demonstratives are words used to indicate specific items or people in relation to the speaker's position. The main demonstratives in English are this, that, these, and those. This and these refer to items that are close to the speaker, while that and those refer to items that are farther away. Demonstratives can function as pronouns or adjectives. For example, in the sentence "I like this book," this acts as an adjective describing the book. In "I want those," those serves as a pronoun replacing the noun. Understanding demonstratives helps clarify communication by specifying which items are being discussed.