From the Super Mario Wiki, the Mario encyclopedia
The Tracker Barrel is a type of Barrel Cannon that appears in Donkey Kong Country 3: Dixie Kong's Double Trouble! and Donkey Kong Land III. It is emblazoned with the capital letter "T" surrounded by two arrows, one pointing left and one pointing right. The "T" stands for "Tracker", while the arrows represent the barrel's ability to track the Kongs horizontally.
Tracker Barrels allow Dixie Kong and Kiddy Kong to cross large horizontal gaps. When jumped into, the Tracker Barrel shoots the Kongs straight up in the air, following them as they move either left or right. Tracker Barrels shoot the Kongs higher in Donkey Kong Country 3 than in Donkey Kong Land III.
Tracker Barrels are found in waterfall levels, which often feature large gaps. In Donkey Kong Country 3 they only appear in Tracker Barrel Trek, which has their name in the title. In Donkey Kong Land III, they appear in three levels: Rocketeer Rally, Footloose Falls, and Rickety Rapids. Oftentimes the Kongs must carefully navigate around Buzzes while being shot out from a Tracker Barrel, especially in the levels Tracker Barrel Trek and Rickety Rapids.