If you detract from something — like an achievement or an opinion or an object — you take away some of its value or diminish it. That dent in the door of your car may detract from its overall value.
The verb detract comes from the Latin word detrahere, meaning “draw away from,” or “take down.” If you detract from someone’s credibility, you reduce — or take down — that person's worth. When you make valid points opposing an argument a politician is making, you detract from her platform. Detract can also describe drawing attention away from something. If the politician then makes a public display of her humanity, she's trying to detract attention away from your arguments.