If you have a poker face, it means that your expression doesn't tell anyone what you're thinking or feeling. Obviously this is useful in poker, because the other players don't know whether you have good cards or bad ones. But a person can have a poker face at any time, in order not to show his or her emotions. There's also an adjective form: poker-faced. Here's an example: "When she heard the news she remained completely poker-faced and simply thanked us for telling her."