To fire another bet after making a bet in the previous round of betting.
If you were the preflop aggressor, it will usually get checked around to the original raiser, and you will have the option to fire a single barrel to take down the pot or try and build a bigger pot when you feel you have the best hand.
A “double barrel” is when you continue with another bet on fourth street (the turn), and when you’re playing aggressive online poker games, it is not uncommon to see players attempt a “third barrel”, both with their big hands and as a bluff.
EXAMPLE “I was thinking about double barreling because a scare card came on the turn which I can try and represent.”