What does replace a noun?

You're asking about what replaces a noun in a sentence. This is called a pronoun.

Here's a breakdown:

* Noun: A word that names a person, place, thing, or idea. Examples: "dog," "city," "happiness."

* Pronoun: A word that takes the place of a noun. Examples: "he," "she," "it," "they," "we," "you," "I," "this," "that."

Examples:

* Original sentence: The dog barked at the cat.

* Sentence with pronouns: It barked at him. ("It" replaces "the dog" and "him" replaces "the cat")

Pronouns make our writing more concise and less repetitive.