From EncyclopAtys
Slow Movement is an affliction; a negative status effect that handicaps the target.
It is also known as Snare.
A creature under the effect of Slow Movement will simply move slower than usual while under the effect. The degree to which they are slowed varies based on what kind of Slow Movement is used.
Players are able to inflict Slow Movement on a target using the Slow Movement spell from the Offensive Affliction Magic skill. This slows movement speed by about 20%.
Players can increase their chances of resisting a Slow Movement Affliction spell by wearing jewelry that raises their Jungle resistance.
Another way to inflict Slow Movement is to score a critical hit on the target's legs with either a melee or a ranged weapon. This Slow Movement is not a magical effect and thus Jungle resistance will not help resist it. Creatures that are immune to Slow Movement spells similarly are not immune to this effect.
Najabs can inflict yet another type of Slow Movement with their spit attack. This also is not a magical effect and thus similarly able to bypass both Jungle resistance and Slow Movement immunity.