Some bullets cause more damage close in than further way, due to the bullet slowing down and losing punch or kinetic energy.  If all of your guns fired parallel to each other, the bullets would spread evenly over the target plane and cause a small amount of damage. 

If all of the bullets converge on one point on the enemy plane then it causes catastrophic damage at that one point. 

Without convergence, hitting a wing on an enemy would mean half of the bullets don't impact and those that do make a few holes in the wing and bounce off the metal spars or fuel tank. 

With convergence, all of the rounds would strike the wing at the same point and several would punch through the wing spar causing it to break and the wing will fall off or bend backward.  At the very least, there will be a gaping hole in the wing and a massive fuel leak.

To set your convergence, first click Hangar on the clipboard menu, then click the Set Convergence button on your clipboard.  You will be moved around to the rear of the aircraft, vehicle, or boat where you can click and drag on the convergence points to set them.