Ought is a synonym for should, and is often used in the same context. It is an auxiliary verb that is used to express obligation, duty, or correctness, typically when criticizing someone's actions. It is also used to indicate advisability or propriety. For example, one might say 'you OUGHT to do your homework' to indicate that it is the right thing to do. In other words, it is used to express an opinion about what someone should do.