Here's why:
* Nouns name people, places, things, or ideas. Examples: dog, park, book, happiness.
* Pronouns take the place of nouns. Examples: he, she, it, they.
* Possessive pronouns show ownership or belonging. Examples: mine, yours, his, hers, its, ours, theirs.
"Their" shows that something belongs to a group of people. For example: "Their house is big."