Bluffing is a major part of games such as poker, but it can also play an important role in roleplaying games. You get to try to deceive another character into believing something which is not true, as a way to gain something you want or avoid conflict you don't want. In other words, it should be the all-purpose go-to skill for social interaction! Diplomacy and Persuasion are for suckers!
Luke: Well, that was a great swamp juice.
GM: Make a Bluff roll.
Yoda: Good, good! More, you must have. Drink, drink!
Luke: Um. I have to go now. I have things to do, trees to see.
Yoda: Oohhh. Lord Elric Fluffypants. Lost Orb. You seek The Lost Orb.
Luke: How do you know about the Lost Orb?
Yoda: Around when the Lost Orb not first discovered, I was! Hmmm! Your mother has it, she does.
Luke: How could you know my mother? You don't even know who she is!
Yoda: Find Vader on the Peace Moon you will.
Luke: Peace Moon? You're crazy! We destroyed that!
Yoda: The new Peace Moon.
Luke: Oh, right. Because the last one wasn't quite peaceful enough.