A bluff was a tactic in which one would attempt to represent oneself as stronger than they actually were. Although it was most commonly used in games like poker, it also had other applications.
In 2266, a group of Balants from the Balant planet attempted to manipulate Captain James T. Kirk into killing Spock while both were outside the ship. When that failed, Balants confronted the Starfleet officers and professed their superiority, hoping it would lead to the surrender of the USS Enterprise. Once back on the bridge, however, Spock deduced from their behavior that the Balants had been bluffing, and that their abilities were far more limited than they let on. (TOS comic: "Skin Deep")