Definition of blocking bet
The first player to write about the concept of a blocking bet was David Sklansky, the great poker theorist. In a nutshell, a blocking bet is a bet you make on the river to go for a low cost showdown. The blocking bet can also be made on the flop and turn, but this is more rare. This bet is often made out of position on the river when a check on our part will prompt our opponent to bet big, which would put us in a difficult position.
Example of a blocking bet
You have A-2 in clubs and are on the river. So far, your opponent has bet on the flop and the turn. The board is 6-9-3-3-A. On the flop, you had a flush draw, but unfortunately, you didn't hit it. On the river, you may not have the flush, but you hit top pair. Your sidekick is mediocre, but your pair might be good. Your opponent is skilled and here he could have anything. You fear a big bet that would put you in a difficult situation. Also, if he has a pair of tens or jacks, for example, he could pass and you would lose some money. You decide to bet 15 in 60, or a quarter. This is a classic example of a blocking bet. For it to be called a blocking bet, it has to be a small bet against the main pot.