To preach is to deliver a sermon or an otherwise powerful message. You may go to church to hear the minister preach, and you may preach tolerance to your friends.
Have you ever heard anyone say, "That's preaching to the choir"? That means you're wasting your words on somebody who already believes what you believe. If you're giving an impassioned speech about recycling to an audience dressed in reclaimed clothing and sandals made of old tires, you're likely to be preaching to the choir. The word preach comes from the Latin prae, "before," and dicare, "declare." When you preach, you're not just talking — you're declaring something with conviction.