From the Super Mario Wiki, the Mario encyclopedia
A Booster Barrel is a type of Barrel Cannon that first appears in Donkey Kong Country 3: Dixie Kong's Double Trouble! and later in Donkey Kong Land III, in which it has the same purpose. It functions like a rocket by utilizing a flame booster below to transport Dixie and Kiddy Kong to a higher area. It has an up arrow painted on it (similar to that of an Arrow Barrel), indicating that it carries the Kongs upward.
When the Kongs jump into a Booster Barrel, the flame beneath it grows slightly while using it. When the Booster Barrel reaches a predetermined location, it blasts the Kongs out. The Kongs can slightly maneuver themselves either left or right, which is necessary for landing on the ground. Upon blasting out the Kongs, the Booster Barrel slowly descends back to its original position. Booster Barrels most often appear in waterfall levels, more specifically Rocket Barrel Ride, as such stages usually require the Kongs to ascend vertically to the top of the waterfalls.
Booster Barrels also appear in DK: King of Swing and DK: Jungle Climber. In both games, they function somewhat differently than in their original appearance, because the Kongs are capable of steering the barrel itself while using it. Booster Barrels also expel a blue flame rather than an orange one.