"Slow" refers to the ability to slow an NPC'ss melee attack speed by a percentage. This can dramatically reduce the ability for the NPC to perform melee damage. Some higher level and/or special NPCs will mitigate the effects of slow, or are outright immune to it's effect. Many slow spells are based against the "save vs. magic" (magic resist statistic), though others are disease or poison based.
Some NPCs also have the ability to slow players' attack speed.