A hamartia is a "tragic flaw" in the protagonist, and it is directly translated as a mistake or error in judgment. An example would be that Oedipus is said to have a hasty temper; that would be a character flaw that could lead to mistakes or errors in his judgment.
Things like pride, and other qualities that may seem like good qualities to have, can turn into a hamartia and can be the downfall of a character. A hamartia appeals to the reader's sense of pity, and it is an example of ethos in literature. A hamartia can be identified with its association to morals, using the protagonist to try to get a message across to the reader.