It is exclusive to the GoldCombat Knife, to which it is integrated. As the name implies, Fast Melee speeds up a melee attack. With this, the player doesn't need to perform a knife takedown animation like usual in Ghosts. Instead, they will perform a slash attack melee like in Extinction mode and previous Call of Duty titles.
Call of Duty: Advanced Warfare
Fast Melee is an attachment available to the Heavy Shield that's unlocked after the player gets 10 kills with the Heavy Shield. It allows the player to melee faster while holding a Heavy Shield.
Call of Duty: Modern Warfare (2019)
"Training in executing melee maneuvers with great speed"