Demonstrative pronouns are used to replace nouns

Nouns like things, places, events, ideas, and animals, or noun phrases and indicate whether they are singular or plural and near or not near to the speaker.

The most common are this, that, these, and those. — *Near : THIS - THAT - THESE - THOSE : far*

Examples

This isn’t mine. (singular, nearby)

This is used to point to a specific object or item that is close to the speaker. It identifies a particular thing in the immediate context, distinguishing it from other items.

Give me that. (singular, not near)

That is used to refer to a specific object that is likely at a distance from the speaker. It indicates a particular item the speaker wants, distinguishing it from other objects.

These are really gross. (plural, nearby)

These is used to refer to specific items that are likely close to the speaker. It identifies a particular group of objects, suggesting that the speaker finds them unpleasant or distasteful.

I forgot to bring those. (plural, not near)

Those refers to specific items that are likely at a distance from the speaker. It indicates that the speaker is talking about particular objects that are not currently present with them.