DPM or damage per minute shows how much damage the aircraft deals on average in one minute with its weapons. The number takes into account weapon reload time and critical damage. A critical hit adds 50% of the normal damage on top of the regular damage. Projectile flight speed is not included in the DPM calculation.
If the aircraft has a squadron size larger than one, the number is multiplied by the number of planes in the squadron. The number is a theoretical maximum and the actual amount of damage is less because of factors like enemy flares and air defense.
While DPM doesn’t tell you the actual performance of a weapon in the battle, it’s useful when comparing the damage output of different weapons.