Pronouns replace nouns within sentenses.For example, if i said " Romeo went to the movies and Romeo got popcorn and a soda" that would be somewhat repititive.It is much easier to read a sentence that says "Romeo went to the moviesĀ  and he got popcorn and a soda. " The pronoun he replaces the noun Romeo.

Indefinite pronouns, on the other hand take place of a noun,but not one particular noun. In other words, the pronoun is there but, the "Romeo" part of the sentence is not.

Ex. Normally during the holidays, everyone leaves campus.
Someone drank all the soda.