To degrade means to reduce the worth of something, as when smoke and pollution degrade the environment. The word can also mean to disrespect or insult: thoughtless comments can degrade a person.
If you whistle at a passing woman, you're being disrespectful — your behavior degrades women. Degrade is often used to mean to insult someone because of their gender or race. Degrade also means to break down. Old paper or photographs can degrade when left in the light — their material breaks down so that they crumble at a touch.