Typically used with: Persuasion.
In short: lie. Characters with ranks in the Bluff Skill are fake as hell. They weasel their way out of trouble with baldfaced deceptions. Most people exhibit physiological responses when they lie, so part of a good Bluff check represents your ability to suppress these signals. If you want to win at cards, stay out of prison, or run for public office, you have to lie like a rug.
A Bluff check can be used to look another character in the eyes and lie to their face. Anybody can try to fib, but a solid Bluff check makes it believable. Bluff can be used to mislead others for many purposes including passage ("I'm the emperor, you fool! Let me pass!"), false innocence ("It was like that when I got here"), or boasting ("I'm dating the prince").
Tell your lie and make a Bluff check. The creature to whom you tell this lie makes a Discern check. If their result meets or exceeds yours, they don't believe you. If your result is highest, they buy it hook, line, and sinker. While you can't retry a failed Bluff check (fool me once…), you can try a different Bluff on the same subject (fool me twice…).
You can lie with your attacks just as well as you can with your words. See the "Feinting" entry in the Tactics section of Chapter 10: Combat. If a character is difficult to hit due to an impressive Evasion Roll, you can make a Bluff check to create an opening in their defense. This takes as much AP as using the weapon normally does. The defender makes a Discern check. If your result is highest, the difference between the results can be added to your next Attack Roll.
You can use this Skill to force any elemental, humanoid, or legendary creature to lose focus and look away. For instance, by pointing adamantly and shrieking "what the hell is that?!" This presents a brief opening for Stealth or Thievery checks while their attention is drawn away. Make a Bluff check opposed by their Discern check. If your result is highest, the other creature looks off toward the nonexistent disturbance and gains the distracted condition for a single round. If your opponent rolls a Critical Failure, they gawk at nothing and gain the wide-eyed condition for one round.