The Habaki is a piece of metal encircling the base of the blade of a Japanese Sword or other bladed weapon. It has the double purpose of locking the Tsuba (guard) in place, and to maintain the weapon in its Saya (scabbard).
A Japanese katana, a Japanese longsword made famous by the Samurai, is drawn by grasping the Saya near the top and pressing the Tsuba with the thumb to emerge the blade just enough to unwedge the Habaki from inside the Saya in a process called "koiguchi-no-kirikata". The blade is then free in the Saya, and can be drawn out very quickly. This is known as "Koiguchi-o kiru", nukitsuke, or "tanka o kiru" (啖呵を切る, "clearing the tanka"). This is obviously an extremely aggressive gesture, since a fatal cut can be given in a fraction of a second thereafter. This manoeuvre, when practiced is known as Iaido.
The Habaki will cause normal wear and tear on the Koiguchi and either a Shim or new Saya may be needed to remedy the issue as it will become too loose over time. Oiling under the Habaki after cutting or once every few months is recommended by removing the Habaki from the sword.