A “blocking bet” is a smallish out of position bet made with the intention of setting a small price for that street. It can be made with either a weakish hand with showdown value or a draw that you would rather not call a sizable bet with. It can also serve the purpose of stopping a bluff.
Mostly, you’re looking to get to showdown or cheaply see the next card. If you checked, your opponent might well bet large and create a difficult situation or losing proposition for you. By betting small you force your opponent to raise in order to bet bigger and therefore reopen the betting, an assumption of risk many players generally don’t want to take. Additionally, you might get some thin value from betting some hands that your opponent would have checked behind you had you checked.
All that said, blocking bets have some issues. Some experienced players will read right through your sizing and raise-bluff, forcing you to pay an even higher price or fold. If you bet rather than check, you have effectively removed the bluffs from your opponents betting range and any value those bluffs would create for you.
Blocking bets work well against weak, uncreative, and nitty players who infrequently raise bluff. Confine the play to those situations, and blocking bets will be a valuable tool in your poker arsenal. But if you get too frisky with them, you’ll end up costing yourself money!