In the Splinterlands gameplay a tank is a monster that is normally played in the first position of the lineup (frontline). Its role is to take enough hits from the opposing team, allowing friendly backline monsters to attack as many times as possible before the tank is killed.
A tank used as the first-position monster only attacks the opposing first-position tank.
Tanks are often very good anti-melee monsters, but sometimes they offer protection against magic or range attackers.
In other situations, a tank also has the Taunt ability, in which case it doesn't matter where it is placed in the lineup, it will attract attacks from all monsters except those which have the Scattershot ability.
Choosing the right tank is very important for the outcome of the game. The wrong tank for a battle can lose a fight all by itself.
If in a game the purpose is to outlast the opponent, it is often the tank that is the most important card to protect.