A postmortem is a medical examination of a dead body. It's a kind of autopsy done to learn exactly how the person died.
In Latin, mortem is a form of the word for "death," and post means "after." A postmortem, logically enough, is something that happens after death, usually an examination. Another type of postmortem is a meeting or discussion that follows the completion of a project — this kind does not involve an actual death, but simply the end of something, like an art installation or a business venture.