From the Super Mario Wiki
Bowser Barrels are objects that appear in the Goomba Takedown challenge in Mario Kart Tour. They are red explosive barrels with spiky hoops and Bowser's emblem depicted on the outer edge. Some of them are found on the ground, while others float in mid-air via propellers. If the playable character drives into a Bowser Barrel, or throws a Bob-omb at it, the barrel will explode, instantly defeating any Goombas in the vicinity of the original location of the explosion.