A melee weapon is any weapon used in direct hand-to-hand combat; by contrast with ranged weapons which act at a distance. The term melee originates in the 1640s from the French word mêlée, which refers to hand-to-hand combat, a close quarters battle, a brawl, a confused fight, etc.
In-game, pressing the F key will do a quick stabbing animation with the melee weapon.
It can be equipped for "blade running" by holding F or hitting 3, from there the player can left click to stab.
It can then be dequipped by scrolling or pressing the 1 or 2 keys (as that will bring up your primary and secondary). Another knifing animation can be done when the player right clicks.