A tank is a role in Dota. A tanks role involves taking damage for allied members. Tank does not always involve having a lot of HP and armour because then the enemy will concentrate on other members of your team and most often, your team carry which is usually very fragile. Therefore a tank has to have an element of their playstyle that forces the enemy to attack them. This could be a taunt such as Axe's berserker's call or a massive AOE damage such as Leshrac's Pulse Nova. In general a tank needs to be both important to kill and hard to kill.