We use a pronoun to refer to a person or thing, that is, instead of a noun.
Jane: Which dress are you going to wear on your date?
Sue: The red dress.
OR
Jane: Which dress are you going to wear on your date?
Sue: The red one.
Instead of repeating 'dress', Sue says 'one'.
There are other uses of 'one'. If you find a sentence where it uses 'one', and you don't understand what it means, post the sentence in the forum and we can explain.
