Root rotting (also known as Root Dotting or Root DoTing) is a strategy for killing foes that revolves around using a root debuff to prevent the target from moving, while simultaneously using damage over time (DoT) spells to reduce its health while standing outside of its melee attack range. When the target's health gets low enough, a simple nuke may be enough to finish it off. If the target is a mob who flees when low on health, it can alternatively be snared and finished off with melee attacks, provided that the root is broken first and the snare is potent enough to keep the mob from moving at all once it starts to flee.
Because of this strategy's nature, it works best against targets with low magic resistance who do not summon and have no meaningful way to deal damage from a distance. It has advantages over kiting in areas in which the available space to run around is limited (for instance, in many indoor zones) or in areas with high mob density, in which kiting may have a high chance of bringing adds. A group that plans to do most of its killing by kiting may occasionally use root-rotting in order to do the initial break in and establish some safe space in which to kite.
Root rotting can be used against some mobs that summon but requires much more advanced tactics than it does when applied against ones who do not. Against a summoning mob, the player or party doing the fighting must be ready to heal the damage the mob manages to cause when it summons, plus continuing to keep the mob rooted while also maintaining enough DoT spells on it to finish the fight before the player(s) run out of mana. Against most high-level summoning mobs, this is a tall order, and may require some degree of overgearing or overleveling on the part of the player(s) in order to be feasible.
A related strategy is root nuking, which functions similarly but involves repeatedly hitting the target with direct damage spells instead of using DoTs. Since nuking has a chance to end root effects prematurely, it tends to be riskier to execute.
Note that neither root nuking nor root rotting can be used against mobs who are immune to changes in run speed, since the root effects will always be resisted. Certain forms of kiting and charm killing may still be viable, however.