What part of speech relates a noun or pronoun to other words in a sentence?

A pronoun takes the place of a noun or nouns in a sentence. Example sentence:

John is my brother, he is a year older than me.

The pronoun 'he' takes the place of the noun 'John' is the second half of the sentence.

Fran and Frank have two children, they have a boy and a girl.

The pronoun 'they' takes the place of the nouns 'Fran and Frank' in the second half of the sentence.