Clankers are magical items which make a loud noise when shaken and are used in Gringotts Wizarding Bank. They are small objects kept in a leather bags and when shaken emit a loud noise.
They are used in high-security to ward off the dragons guarding them. The dragons associate the sound of the Clankers with pain and so flee the vault that they are guarding when they hear the sound. The creatures are "trained" to expect torment whenever they hear the sounds produced by these devices, causing them to retrreat from the source. They are used by the goblin employees at Gringotts in order to access certain vaults.
In 1998, Harry Potter, Ron Weasley, Hermione Granger, Griphook, and Bogrod (who was Imperiused at the time) used the Clankers to ward off the dragon guarding the Lestrange Vault, in order to get to the Horcrux inside.