Is a pronoun a word that replaces a noun?

Yes, pronouns are Words such as he, she, it, they, them, etc.

A pronoun is used as a substitute for a noun or a noun phrase, e.g.

She was in the garden

Anything can happen

Did you see that?

It's lovely weather

Using a pronoun often avoids repetition, e.g.

I found Ryan - he was in the yard (instead of Ryan was in the yard)

Where are your glasses? - I've got them (instead of I've got my glasses).