A float is a move where an aggressor is called so that a player may retake the initiative in a later round, provided that the original aggressor allows it. It is a broad term. The call on the flop can be a bluff call, smooth call, or a call for pot control. The term is often used in sense of a bluff.
Float as a Bluff
One does not need a made hand to execute a float, even though the expected value will be elevated when one possesses a draw with which one could win the showdown if it were completed. Calling a continuation bet can be done as a pure bluff to represent a strong hand or slowplay or to show an opponent that the pot will not be easily won and to convince him to give up his bluff.
Many players avoid making a bet in the third betting round (a so-called second barrel) without a good made hand if an opponent calls two bets in spite of that player's aggression. A float is therefore useful when a player is in position to give up his hand if it does not improve after his opponent has called a continuation bet. Against players who often make a third bet, a float is useless. It is also useless if an opponent only makes a continuation bet when he has a good hand. Players who often make a continuation bet after a raise and then fold when their hand does not improve are ripe for floats.
A typical floating scheme in Texas Hold'em goes like this:
Preflop: Player A raises, player B calls.
Flop: A bets, B calls.
Turn: A checks, B bets, A folds.