A blocking bet is a smaller that normal bet used to see a cheap showdown. It’s an attempt to block an opponent from making a larger bet when checked to them.
An example of a blocking bet.
Action is folded around to you pre-flop. You limp in from the small blind with JTo, your opponent raises from the big blind pre-flop, and you call. The flop comes down A–J–2. You check, and he makes a continuation bet, which you call. The turn brings a 5, you check, and your opponent checks. Now the river brings a 8, and you are not sure if you have the best hand. If you do, you know your opponent wont call a large bet, so you decide to make a smallish blocking bet on the river to try and get some thin value from weaker pairs and get to showdown cheaply.