A barrel poker bet is when a poker player (in Texas Hold’em or Omaha poker) makes successive bets on post-flop streets.
Most often, it’s used in the context of a “double” or “triple barrel”, referring to a string of bets.
Let’s look at this through a simple example. For instance, you know that you have a good hand after the flop and decide to bet the pot.
After the fourth street, you continue with the same tactic and bet the pot again. As the river brings up the last card, you bet the pot again, effectively making the “triple barrel” bet throughout the hand.