A barrel or vat was a container of a cylindrical shape used to store liquids.
Jadzia Dax brought three barrels aboard the IKS Rotarran in 2373. After they returned from rescuing the crew of the IKS B'Moth, General Martok asked Major Kira to have several barrels waiting for them when they reached Deep Space 9. (DS9: "Soldiers of the Empire")
In 2374, Benjamin Sisko made a bet with Martok that whoever set foot on Deep Space 9 first would be owed a barrel. Sisko was the first to board DS9, but told Martok they would share the barrel together. (DS9: "Sons and Daughters", "Sacrifice of Angels")
Before the Battle of Cardassia, Sisko speculated that Martok was bringing a bottle of bloodwine with him to celebrate their eventual victory. Martok responded that he had brought not a bottle, but a barrel. (DS9: "What You Leave Behind")