A barrel tap is described in game as "a rounded tap, to place on barrels for wine and beer." It is made with the Carving skill (level 3 or higher), using a knife of iron or better quality. It is used to seal a Wine Barrel after it is full of Grapes (see the Viticulture Guide).
1 wood = 1 Barrel Tap with a 120 second Focus timer between whittles.
"You carve and hollow a rounded tap, to place on barrels for wine and beer."