From the article:
The IS-2 heavy tank was designed to replace the aging KV tanks as the Guards heavy breakthrough units. It was equipped with the massive 122mm gun; ideal for hunting the enemy heavy tanks and firing HE (High Explosive) rounds into fortified enemy positions.
The IS-2 obr 1944 introduced a faster-loading version of the gun, the D25-T with a double-baffle muzzle brake and better fire-control. It also featured a simpler hull front without a step instead using a flat, sloping glacis armour plate.
Sometimes referred to as the IS-2m (lowercase 'm'), but this official designation was not used until the modernisation of the tank by the Soviets in the 1950s (IS-2M, uppercase 'M').