By your biding you pretend that you have good cards but infact you have nothing. If your bluff is successfull all your opponents will fold because they think they have no chance to win and you get to win the blinds.
Beware: you can only bluff if there are bluffable cards on the flop.
If the flop is i.e. 9, 10, K you could bluff a straight. If it is 2,6,10 you can go for a three of a kind bluff maximum. Always check if a extremly good hand is possible before you bluff.
It is also possible to bluff a bluff. If you have a extremly good hand (full house or straight flush for example) and you want to make the most out of it you can bluff a bluff. Your opponents should think, that the noticed your bluff and go with your extremly high raises. This is rather difficult to achieve but if you failed a bluff before it is possible, that the trap works.
|Poker - Cheatsheets#2 Bluffing|