A face lacking any interpretable expression, as that of an expert poker player.
A poker face can be the difference between winning and losing in live poker games. A persons face may have many tells, the idea of keeping a poker face is to eliminate the possibility of showing signs of what you maybe holding in your hand, how strong your hand is and of course how weak.
Many people cover their eyes with sunglasses, wear caps and look directly down at the table when playing in hands. This is a form of keeping your face hidden and hence a good way to assist in a poker face.