Possessive nouns are nouns which possess something; i.e. they have something. Of course, the thing the noun possesses is another noun. You can identify a possessive noun by the apostrophe; most nouns show the possessive with an apostrophe and an s.
The cat’s toy was missing.
The cat possesses the toy, and we denote this by use of ‑‘s at the end of cat.
Is this Brandon’s book?
I pulled a feather from the goose’s tail.
If the noun ends in an s, we generally use only an apostrophe after the noun, although it is acceptable to use ‑‘s. The problem is that it looks funny with all those Ss, which would detract from reading.
I’ve been invited to the boss’s house for dinner.
I’ve been invited to the boss’ house for dinner.
The trainer flipped a fish into the walrus’ open mouth.