In the movie, a family moves to a new home near a pet cemetery. Their cat, Church, is accidentally killed by a truck and buried in the cemetery. However, the cemetery is cursed, and Church returns from the grave as a twisted, evil creature.
This is a horror story designed to scare the audience and explore themes like grief, death, and the consequences of tampering with the natural order.
There are no actual reports of pets returning from the dead in a malevolent form, so the story of the cat in "Pet Sematary" is purely a work of fiction.