Caulking is both the processes and material (also called sealant) to seal joints or seams in various structures and some types of piping.The oldest form of caulking is used to make the seams in wooden boats or ships watertight, by driving fibrous materials into the wedge-shaped seams between boards.A related process was formerly employed to join sections of cast iron sewerage pipe.he oldest form of caulking is used to make the seams in wooden boats or ships watertight, by driving fibrous materials into the wedge-shaped seams between boards.