A hero that is a 'Tank' is a playable character whose role is to absorb damage and create mayhem in the enemy's lines. Generally, these heroes perform best with items that increase health and survivability, and naturally have higher defensive stats than other heroes.
Most tanks also come with at least 1 ability that can be used as CC - Crowd Control, such as a stun or slow, effectively forcing an enemy to attack them since they cannot reach the targets they want to.
While tanks are usually not very effective damage dealers, they can often still win in single combat against a more offense orientated Hero simply because of how long it takes for the Tank to die.
Tanks should generally be played in the front lines of your team. Because of a tank's typically low damage and high resistances, if they are in the back line and are never damaged, the majority of their usefulness is wasted.