The 'Hidden Blade' is a tight gauntlet that wraps around a person's left forearm, with a complex mechanism inside that deploys a short, incredibly sharp blade for use in quietly killing an individual. The 'Hidden Blade' is a signature of the Assassin's Creed series, where it is used by both Altair and Ezio Auditore, as well as their descendants Connor Kenway and Desmond Miles.
In Altair's time, the hidden blade had an almost religious significance to the Brotherhood of Assassins; upon reaching full membership in the Brotherhood, an Assassin's left ring finger is amputated halfway down, to leave a gap in their fingers where the blade will protrude when the mechanism is activated. This necessarily weakened the grip of their left hand, but Assassins were taught to compensate.
In Ezio's era, the Assassins had taken the more practical method of redesigning the device so that the blade is deployed in front of the palm. Not requiring Assassins to cut off their fingers not only strengthened their grip, it was much easier to hide from prying eyes. Instead, full recruits receieve a brand of the symbol of the Brotherhood on their left ring finger, which can easily be covered up by gloves or a large ring.