It means a person's heart stopped, and doesn't say much of anything. If a physician is treating a person with a terminal illness this is a common statement, however without an autopsy it is impossible to provide even an educated guess.
The heart may stop for a number of reasons, and go into ventricular fibrillation (quivering like jelly and not pumping), due to cumulative damage, toxins, the ph of the blood, or a number of other reasons.
When the heart stops breathing eventually stops.