It's the implementation of the force of gravity into the game environment. If you shot artillary in real life, or even a bow and arrow for that matter, gravity will pull the projectile down toward the ground as it passes along its trajectory. It's just the simple physics of life
All the anti-armor shells have the gravity effect so once you master one type, like a tank barrel, all the rest (artillary, bazookas, etc.) will follow. Also remember that one antitank shell to the rear of a tank will do the most damage, most times destroying it with one shot. So hit the enemy tanks in the ass and start taking names.