A more thorough explanation:

Definition: Occurring or existing after death; especially, (of a child) born after the father's death.

Examples:

• After the author's death, a posthumous book was published using his unfinished manuscripts.
• The artist's posthumous exhibition showcased his final works.
• The king's posthumous child was not eligible for the throne.

The first two examples illustrate the general meaning of posthumous, which refers to something that happens or exists after someone's death. The third example specifically refers to a child born after the father's death, which is a common usage of the term.