To denounce is to tattle, rat out, or speak out against something. When you stand on your desk and tell the class that your partner is cheating, you denounce him or her.
The prefix de- means "down," as in destroy or demolish (tear down). Add that to the Latin root nuntiare, meaning "announce," and de- plus announce equals denounce. It's a word that shows up in the headlines often, as a country might denounce a corrupt election. Politicians love to denounce the shady behavior or their opponents. Denounce can also mean the official end of something, like a treaty.