A tragic flaw is a literary term that refers to a personality trait of a main character that leads to his or her downfall. In other words, a character with a tragic flaw is in need of some kind of attitude adjustment. The term usually comes up when you're studying a tragedy — that is, a piece of literature in which the main character ends up dead or otherwise defeated. In this kind of story, the main character is sometimes also called the tragic hero.
Some examples of classic tragic heroes are Hamlet and Captain Ahab. Both of these guys had to deal with tough decisions and strange twists of fate, and they end up dead by the end of the story. They're such complex characters that people are still debating their tragic flaws even to this day. But before you tackle one of the big guys, let's start with something simple.
Think about the hare in the fable The Tortoise and the Hare. It's not necessarily the best example of a tragedy, but things do end up badly from the hare's point of view. We know that the hare is capable of running the race much faster than the tortoise, and yet he ends up losing. So, what is it about the hare's personality that causes him to lose? Sure, you could list all of the individual bad choices he made along the way, but ultimately the hare's tragic flaw is his arrogance. He was so convinced of his superiority that he ended up sabotaging himself.
Now ask yourself what underlying personality trait is causing the tragic hero in your play or novel to self-destruct. Figure that out and you've got yourself a tragic flaw.