When a member of the clergy preaches a sermon, he or she gives a talk on a religious or moral subject during a religious service.

When people preach a belief or a course of action, they try to persuade other people to accept the belief or to take the course of action.

If someone gives you advice in a very serious, boring way, you can say that they are preaching at you.

If you say that someone practises what they preach, you mean that they behave in the way that they encourage other people to behave in.

If you say that someone is preaching to the converted, you mean that they are wasting their time because they are trying to persuade people to think or believe in things that they already think or believe in.