Leashing is a game mechanic which forces players to fight an enemy within a certain radius of their spawn location. In ESO, most mobs are leashed to a specific area. Attempting to lead a mob out of this radius (such as by taunting) will cause them to cease combat and return to their starting position. When a mob ceases combat, it will restore to full health and become immune to all damage until it returns to its initial position. Such mobs won't engage in combat with any other players along the way.
Mobs will follow players for a short distance outside their leashing radius and continue to attack. This can prove dangerous, but a single attack will force them to cease combat. Once a player gets far enough away, the mobs will cease combat of their own accord. In addition to a leashing radius, instanced mobs may outright disappear and respawn if led outside the instanced area.
Leash mechanics are important to bear in mind if you plan to run past many mobs. This will result in a large following of hostile enemies, but most will give up pursuit when attacked after a certain distance. The mechanic may also prove disastrous if triggered during a boss fight, since it will reset the boss's health. For instance, the Spawn of Mephala boss in Fungal Grotto II has a small leashing radius to prevent exploits.