Generally, a main tank is first of all a tank. Beyond that, there are three possible definitions, which are all related to each other, but don't quite describe the same thing:
- In a raid, the main tank does most of the pulling and keeps the hardest hitting or most difficult mob(s) on himself.
- In a broader sense, a main tank is the player in a party that concentrates on keeping the mobs attacking him and only him, while the other players pick them off.
- A player acting as a tank becomes a main tank when he is equipped and experienced enough to allow the other members of the group to perform at the limit of their ability.
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 four combinations of spec and class which can fulfill the MT role: Protection warrior, Feral druid, Protection paladin, and death knight. There are many other combinations of class and spec which can serve as 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 and since TBC, two other specs became viable main tanks. Thus 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 is prepared to tank but only in rare situations (e.g. the main tank is dead) is known as off-tank.