Definition 2 barrel poker
When the player who raised preflop (we'll refer to this as the player with the lead) bets the flop and turn, he is said to have made a double barrel or 2-barrel (we could also refer to this as a c-bet turn or continuation bet on the turn). You can make a single (flop), double (flop + turn) or triple (flop + turn + river) barrel. The term "barrel" will generally be used only for the player who has the lead, i.e., the player who made the most aggressive action on the previous street (the preflop raiser for example).
Setting up the 2 barrel
A player on the button opens preflop to 3 big blinds when the players before him have all folded. The player in the small blind calls and the player in the big blind folds. Here, the button has the lead and will be able to barrel. The flop comes K-A-4 in spades. The small blind checks and the player on the button makes a bet which is called. The turn is a 5 of clubs and the small blind checks again while the player on the button bets. So here we will say that he will have made a double barrel or 2 barrels.