Zeroing means a weapon gets better accuracy if they keep firing on the same target several times in a row. This zeroing mechanic applies to machine guns. Thus if you want best accuracy, have your gunners to fire in short controlled bursts.

Ah thanks!

For the real life definition: It means setting/calibrating the sights of the firearm to the target. That's also why you have adjustable iron sights in many rifles and machine guns, and such.