It's a term from Ancient Greece, the birthplace of theater. It means excessive arrogance or pride, especially that which brings down a hero. Oedipus Rex displayed hubris when he over-confidently assumed that he could never have done something so terrible as kill the Theban King Creon and accused the seer Tiresius of lying when he said he had. It is a primary ingredient in all tragic drama from the period.
Allow me to offer a more modern example. A woman posts an entry on her blog telling the world about how she's losing weight and about the food rules she has set up for herself that are making that possible. Complimentary remarks ensue. Self-congratulatory feelings prevail.
And then said woman makes a batch of Golden Graham S'mores. Golden Graham cereal and mini-marshmallows coated with a chocolate/corn syrup/margarine blend. Absolutely decadently delicious. This woman was well aware of her personal weakness for this dessert. She knew that she had a history of devouring this dish in short order, sometimes straight from the mixing bowl, with no regard for the consequences to follow. But this time, she allegedly made them for her family -- and it really was her intention that everybody else eat their share. Apparently, they were not as enamored of the dessert as she was, however. Within 48 hours, the bulk of the batch has been consumed by said woman, much to her shame.
Just figured I'd better come clean. Confession is now made, and mercy is new every morning. Hallelujah, and amen. Pass the carrot sticks.