The describe the term "off-tanking", one must be familiar with how a raid in EverQuest is typically run.
A warrior, by virtue of the highest hit points, a plate class with the highest armor class, and best damage mitigation, is designated as a "main tank". DPS classes and healers assist the main tank in killing his targets (when assistance is called) and in keeping him alive. During boss encounters, a secondary and even a terciary tank may be designated should the main tank die.
Consummate with the difficulty of a raid zone, it is rare for the raid to be able to fight just one mob at a time. Off-tanking is basically any tank-class (warriors, paladins, and shadow knights) tanking the extra mobs that spawn and aggro while the main tank (with the assistance of the rest of the raid) kills his targets.
Off-tanks are a very necessary part of a raid, as chain, leather, and silk (cloth)-wearing classes have a drastically reduced ability to mitigate damage as compared to tanks, and fewer hit points. Their advantage, however, is that they are able to do more damage per second (DPS) than tank classes.