2 Barrel poker definition.
When the player who raised pre-flop (we will speak here about the player who has the lead) will put the flop and the turn, we will say that he made a double barrel or 2 barrels (we could also speak about c-bet turn or continuation bet on the turn). You can make a single barrel (flop), double (flop + turn) or triple (flop + turn + river). The expression "barrel" is generally used only for the player who has the lead, i.e. the player who did the most aggressive action on the previous street (the pre-flop raiser for example).
2 barrel's scenario.
A player on the button opens preflop to 3 big blinds while the players before him have all folded. The player on the small blind calls and the player on the big blind folds. Here the button has the lead and will be able to barrel. The flop falls K-A-4 while 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 check again while the player in position on the button bets. Here, it will be said that he will have made a double barrel or 2 barrels.