Tank swap is term used for a boss in a raid which requires two tanks to beat. This mechanic usually comes into play when a debuff on the tank makes it impossible to keep tanking. This might include some detrimental effects like immense damage over time, reducing the tank's healing and survivability, or even killing the tank outright after certain conditions. To allow for the original tank to reset their debuff, the other tank taunts the boss to take aggro off the first tank.
There a variety of possible mechanical variations. The tank swap debuff may be applied over time, or after certain boss abilities have occurred. Some of these debuffs may stack if multiple casts have been taken. On the other hand when removing the debuff, generally the debuff will expire over time. However if the debuff does not expire, then a specific action or movement to a certain position be required clear the debuff. The original tank is free to perform such responsibilities to clear the debuff once the new tank has aggro.
Historically, prior to the introduction of tank swap mechanics many raid encounters simply had a maintank holding aggro on the boss, with an offtank focusing only on adds, or just as a backup tank. The main boss itself did not usually need a second tank at all in single-boss encounters, and a full co-tank would only be required for the whole fight if the encounter had a pair of bosses, such as Twin Emperors or Eredar Twins. The swap mechanic was largely introduced during towards the end of Wrath of the Lich King and formalized in Catalysm and standardized raid compositions to always require two tanks. In Ulduar, Algalon the Observer had the "Phase Punch" ability that teleported away a tank and forced another tank to take over tanking the main boss. As examples in Icecrown Citadel, Deathbringer Saurfang had "Rune of Blood" which would heal the boss until the tanks swapped; Festergut would instantly explode the tank if they took too many stacks of "Gastric Bloat". Whereas in Cataclysm with Blackwing Descent and the Bastion of Twilight, bosses had abilities explicitly labeled as tank responsibilities in the adventure guide. Some examples include Halfus Wyrmbreaker's "Malevolent Strikes" reducing healing received, or Chimaeron's "Break" debuff massively increasing physical damage taken. Eventually in later expansions, almost all newly introduced bosses would have some form of tank swap.