Runner(s) usually refers to a mob or mobs that are escaping from a battle engagement (usually from you and/or from your teammates) because of being either low on health points or feared. This is most common in Humanoids and is prompted by the emote "<mob> attempts to run away in fear!"
The main problem with runners (especially in instances) is that if they run close enough to other mobs, they will alert them, causing the additional mobs to attack your party. This can easily result in a wipe, especially if the party is already low on health/mana or were already facing sizeable opposition.
However, while runners can and often will alert other nearby mobs, as the path of the runner is relatively randomly determined, it is quite possible, especially in empty areas (or areas that have already been cleared of mobs) for runners to fail to alert any additional mobs. In this case, they will run around for a period of time, and then walk back toward their previous target and resume attacking. This is rarely of concern, as the mob by this point is usually very low on health, and is easily killed. However, players will still be considered 'in combat' until the mob has been killed, or otherwise dis-engages from combat, meaning that certain abilities (such as ) will not be usable. In situations where runners are not problematic, especially in critical encounters when there are other targets to deal with, it may be preferable to let the target run, rather than waste time and resources killing it immediately. In this respect fleeing can be considered a free fear effect.