Runner(s) is usually referred to a mob(s) that is escaping from a battle engagement (usually from you and/or from your teammates) because it is either low in health points or feared. Usually, you do not want runners (especially in instances) because they call other nearby mobs for assistance. To prevent runners, players try to finish the mobs with low health points as quickly as possible or to snare/stun/root them long enough to kill them before they call for more help.
The warlock spell 
[Curse of Recklessness] prevents a mob from running, as does the paladin with 
[Judgement of Justice].