Nouns are people, places, and things. That's their form, what they look like. They also have a function, which means what they do in a sentence. Nouns have four basic functions:
1. As a subject
Ex: The box is red.
=> Subjects usually come before the verb.
2. As an object
Ex: I see the box.
=> Objects usually come after the verb.
3. As an object of a preposition
Ex: Look at the picture on the box.
=> Preposition describe everthing a cat can do; e.g., a cat can go in, on, over, under, beside, next to, etc.
4. As an adjective
Ex: My doghouse.
=> Adjectives answer the question what kind of?; e.g., what kind of house? A house for a dog.
Regular noun: box (it's a thing)
Subject noun: The box is red. (it functions as the subject of the sentence.)