A barrel is a container of a cylindrical shape. Klingon bloodwine is often transported in barrels. (DS9: "The Sword of Kahless"
Jadzia Dax brought three barrels aboard the Rotarran in 2373. After they returned from rescuing the crew of the 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")
In late 2375, Worf gave several barrels of bloodwine to several crewmembers of the Koraga, before their mission to the Badlands. He did not pay for them at the time. (DS9: "Penumbra")