A pronoun takes the place of a noun.
Example story:
Mary is one of the heads of the ToJi Corporation. Mary works with Mr. James and Mr. James' son Tom. Mr. James and Mr. James' son Tom are experts in biochemistry. Mary, Mr. James, and Tom researched and invented a drug for cancer treatment.
If the story above is written using pronouns:
Mary is one of the heads of the ToJi Corporation. She works with Mr. James and his son Tom. He and his son Tom are experts in biochemistry. They researched and invented a drug for cancer treatment.