Floating is a move that is used by many experienced poker players to win pots against weak opponents. It's a very good tool in each poker player's arsenal if used right.
Floating means calling the flop with a weak hand that you normally wouldn't, because you don't believe that your opponent have much of a hand either and will shut down the betting on turn, enabling you to steal the pot with a bet. Many of the players on the low and mid stakes online are scared to fire a second bullet on the turn without a pretty strong hand because they believe that you will need to have a hand to call them on flop.
The float play works best against weak and in-experienced players. As with all bluffs it shouldn't be made to often on order to be successful. This move is usually made on the turn rather than the river. You don't want to give your opponent a free card that will make him/her call your river bet.
So what is a good spot for floating?
You are acting on the read you have on your opponent so position is important here. You want to see if your opponent checks or fire another bullet on the turn. (Because your move basically depends on a check here.) To be floating you want to be up against a single opponent. There is a too great a risk that someone will hit a good turn card if you are up against multiple opponents, making it impossible to bluff.
To have a good read of your opponent is essential. The ideal candidate to be floating against is a player that usually continuation bets on the flop, but shuts down on the turn if someone calls. Watch out for strong opponents that will continue to push and weak opponents that never fold if they've got any piece of the board what so ever.
The texture of the board is also important; you don't want it to contain any good draws. Your opponent might be on a draw and less inclined to fold, but more importantly he/she might think that you are on a draw and therefore check call the turn instead of folding. What you want to avoid is flops with many high cards, connected flops and flops containing two cards of the same suite.
Floating is all about timing, so try to choose your spots careful and don't over do it. If you've read the situation wrong and your opponent keep betting on the turn, it's probably best to shut it down and wait for a better opportunity.