A block bet is a bet made primarily to stop your opponent from bluffing, or betting more than you are happy to call. When used properly they can be effective, but it's important to be aware that they have two big drawbacks. They are most commonly used on the river when out of position.

A) They are very exploitable, by either good or just aggressive opponents

B) If they work they will often stop your opponents bluffing. This will cost you money if you intended to call a bluff.

 

If you are playing against somebody who will recognize a block bet, they will often raise you as a bluff. Even the weakest players will recognize small bets as defensive bets, so its important to bet enough so that you don't look too weak, this usually means at least ½ of the pot.

The best time to use them is when the pot has grown to a much larger size than you are comfortable with for your modest holding. So a single pair of aces may be worth a half pot bet on the river, as your opponent will nearly always only raise if he has a monster hand, so you save money against two pair or trips but get to a showdown. As by checking on the river you are signaling the fact that you don't have a good hand, and your opponent will know he can value bet his strong hands big. Note that against bad players who tend to bet small in these situations, or against very tight players who will not bet unless they have you beat then check calling or check folding is better. Against aggressive players who bluff too much you should check call all of your marginal hands on the river.