The hunter bracelet is a piece of jewelry linked to a tattoo given to an ex-human vampire in order to tame them. The hunters perform a ceremony using the blood of the ex-human onto the bracelet. The bracelet holder then only has to touch the bracelet to the tattoo in order to disable the vampire.
It is unknown whether taming only works on ex-human vampires. It is not confirmed as to whether the bracelet will work for anyone wearing it, or only the individual who partook in the ceremony, it is most likely the latter as Yuki is known as the one who has tamed Zero.
Possibly with the aid of Yuki's Pureblood powers, the bracelet can also take away/seal selected memories in Zero; however, they appear to be able to be unlocked if the right event triggers them.
There are probably other types of jewelry that can be used in the ceremony, but only Yuki's bracelet has been seen in the manga and anime.
Following the destruction of the first Parent Metal, the bracelet lost its power. Although it was therefore rendered useless as a taming device, Yuki chooses to keep wearing it, stating that it reminds her of the promise to not kill Zero and of her wish for him to smile peacefully someday.