A possessive noun is a noun that possesses something—i.e., it has something. In most cases, a possessive noun is formed by adding an apostrophe +s to the noun, or if the noun is plural and already ends in s, only an apostrophe needs to be added. In the following sentence, boy’s is a possessive noun modifying pencil: The boy’s pencil snapped in half. It is clear that the pencil belongs to the boy; the ’s signifies ownership.

The cat’s toy was missing.

The cat possesses the toy, and we denote this by use of an apostrophe + s at the end of cat.