Answer:
Most simply, a tragic flaw is the characteristic of a protagonist, or "hero," that leads to his downfall.
Some of the most well known examples of tragic flaws are in Shakespeare's tragedies. The main character is strong and capable, and seems to have everything going for him, and as such, the reader or audience generally supports and sympathizes with him. However, one major flaw causes him to self destruct. This is what is so tragic about the tragic flaw - such an admirable, reliable character is ruined by one trait.
Examples include Macbeth, a high ranking general with a great deal of power, whose downfall is an ambition that pushes him into behaving immorally and ultimately being unable to bear the psychological impact, as well as the physical retributions. Thus it is Macbeth's ambition that becomes the downfall - or, the tragic flaw - of such a powerful character.
to put it more simply, its a character flaw in another wise noble character resulting in a fall usually death