Generally, a main tank is first of all a tank. Beyond that, the term usually carries a few different meanings, depending on context:
- In a raid, the main tank can refer to the tank who does most of the pulling and keeps the hardest hitting/boss mob(s) on himself. This is the traditional definition of a "main tank" and still applies today.
- In a tank-swap scenario, the tank currently tanking the boss is referred to as the main tank. Often this will be used in reference to healing assignments; for instance, a healer may be assigned to the "main tank" if he is assigned only to heal player currently tanking the boss.
- In a guild/role context, a main tank can refer to a player whose primary role in a guild is to play a tank. This can be used in contrast to DPS/healers with a tanking off spec (referred to as off tanks).
The reason why you need a main tank is that when fighting strong mobs, often the healers and damage dealers will be killed with only a few hit from the mobs. Even if they can keep themselves alive with crowd control abilities or healing spam from the healers, their own healing or damage dealing job will be affected. Therefore, the most effective way when fighting those mobs is to have a high armor and stamina main tank to keep the aggro of the mobs on him, so that the healers and damage dealers can concentrate on their own job.
There are currently five combinations of spec and class which can fulfill the MT role: Protection warrior, guardian druid, protection paladin, brewmaster monk, and blood death knight. There are many other combinations of class and spec which can serve as a tank, but none of them deserve the label "main tank".
Prior to the release of the Burning Crusade, only warriors were truly able to main tank, leveling a warrior was tedious and learning the tanking skills was (and is) difficult. In consequence, there were only a few outstanding players who reached the status of main tank. Since at the same time a tanks job is extremely important, highly skilled and well equipped and warriors were highly valued. The importance of a main tank was such that some raiding groups simply dissolved after their main tank quit.
With the later patches, four other specs have become viable main tanks, and as a role tanking has been made far more engaging. Hence main tanks aren't as scarce as they used to be, but the main tank is still an extremely important member of any raid or group.
Someone who tanks less or weaker mobs to take pressure off the main tank is known as the off tank.