From the Super Mario Wiki
Basins are items in Mario Kart Arcade GP, Mario Kart Arcade GP 2 and Mario Kart Arcade GP DX that lock on and whack the opponent's head. Although a basic item, it damages the opponent like a Thwomp, stopping the targeted player and allowing other players to pass by. The item is based on a common Japanese slapstick gag of basins like this falling on people's heads.
Basins can come in singles and in triples, under the name of Triple Basin which can be unlocked as a Prize Item.