A barrel is a big container used to store liquids. If you're moving a barrel, it's best to tip it on its side because its shape bulges at the center, so that it's easy to roll.
Barrels hold liquid in them — anything from wine to beer to oil. A barrel can also be the tube in a gun that a bullet travels through. You might hear the expression "stare down the barrel of a gun," which means you're looking at a gun pointed right at your face. A barrel is also used as a unit of measure or to refer to a large unit of measure. When someone says they are having "barrels of fun," they're having a really great time.