To decrease the importance, value, or effectiveness of something.
Example:
None of the gossip in the new biography detracts in the least from her greatness as a writer.
With the prefix de-, meaning "away," detract means "draw away from." A fact that doesn't match up with the rest of the prosecution's case detracts from it. Richard Nixon's involvement in the Watergate coverup was felt to detract so seriously from his ability to carry out his presidential duties that he had to resign, especially after his detractors had impeached him. (Don't confuse detract with distract, which means "take attention away from.")