A Barrel is a type of storage container. It is often used at times as an Improvised Bathtub. Usually water is poured into it and the barrel is held up over a fire where the water can be heated. This is very common in poor environment's with limited supplies.
Son Goku, Son Goten and Trunks have a bath in the barrel at Goku's house. While Goku is easily beating them in a playful fight, Goten and Trunks both get out of the barrel and fuse into Gotenks and get back into the Barrel. The barrel gets destroyed from the commotion as Goku turns into a Super Saiyan.
Negi Springfield bathes in a barrel after getting some ninja training from Kaede Nagase. Kaede Nagase then Strips naked and gets into the barrel with him. This seems to be Kaede Nagase's way of bathing during the times she is camping for her ninja training.
After the main characters (Oga and Beel) go missing, they reflect that they took 10 different baths in a health club in the demon world, and one of the places they are shown bathing (besides a hot spring, hot tub, pond and waterfall) is a pair of barrels.