Basically speaking, a bluff in Poker is a Bet designed to make the opponent think that you have a good hand. The Player in question raises the bet or even goes All-in to get the opponents to drop out of the game.
The chance with this approach is to win the Pot, although you yourself have comparatively (very) bad cards. The Risk, on the other hand, is that another player will Call the bluff or have such good cards himself that he will call and the "bluffer" will then lose a large amount.
So when a player's primary goal is to get better hands from opponents to fold, that is called a bluff. This is the case even if some of the worse hands call and/or some of the better hands do not fold at all.
Basically said, by the way, can be distinguished between different "bluffs". On the one hand, there is, for example, the bluff with zero pot equity. In this case, the poker player usually has no chance of winning at all, so he bluffs completely. So-called semi-bluffs, in which there is at least some pot equity, are more recommendable. Bluffing on the flop or turn is only recommended if the opponent tends to fold very often and is easily intimidated. Bluffing carries a very high risk, but can also lead to winning the pot despite a bad hand.