Weakness

The flop shows J-7-6. Two players check. Then someone places a bet that seems like about the right amount for either a pair of Jacks or a pair of 7's.

In late position, a player throws out a bet more suitable to a straight, a set of jacks, or a pair of pocket Queens. Or (a bet) considerably larger.

You're sitting there with nothing. What are you going to do. Fold, of course.

What will the player holding a pair of two's or sevens do? Fold.

Will a player holding Jacks call? Would you, faced with a bet that suggests a pocket pair or better?

Maybe all the bluffer has is a Jack, but with an Ace kicker. If you're holding J-4, are you going to risk that possibility?

Hold 'Em is a positional game, and in late position, with other players having demonstrated weakness, this is a prime time to bluff.