Simple Definition:

A relative pronoun is a word that is used to connect a clause [group of words with a subject and a verb] or phrase [a smaller group of words] to a noun or pronoun. It helps to give more information about the noun or pronoun.

For example:

• The boy who is standing over there is my brother. (In this example, “who” is the relative pronoun connecting the clause “is standing over there” to the noun “boy”. It gives more information about which boy is being referred to.)
• The book that I borrowed from the library is very interesting. (In this example, “that” is the relative pronoun connecting the clause “I borrowed from the library” to the noun “book”. It gives more information about which book is being referred to.)

Very Simple Definition:

A relative pronoun is a word that helps to give more information about a noun or pronoun.

For example:

• The boy who is my brother. (“Who” tells us more about the boy.)
• The book that is very interesting. (“That” tells us more about the book.)