This refers to the American card game of poker, where it is important to not let opponents infer the cards you're holding by reading clues in your facial expressions. Generally, one should keep a "poker face" while playing poker, that is, a completely expressionless face, which prevents opponents from seeing any clues about what cards you're holding by interpreting your facial expression. However, some poker players keep a constant smile on their faces at all times, yet it is actually an expressionless smile, a type of "poker face," from which one cannot read any clues to the cards being held.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.