Leading means that you have a vector target that moves around the enemy ship, depending on their speed and direction. With moving target you have to "lead" - aim in front of them so the projectiles have time to travel there. I think you are familiar with this tactics from shooting ducks and whatnot.
So Leading - you are trying to aim at the vector indicator in front of the enemy ship (that indicates where in space the enemy will be once your bullets arrive there).
Trailing on the other hand is tied to movement of YOUR ship as well as enemy. As you aim around, you are "dragging" the crosshairs around with a delay, that indicates where would you hit, if you shoot now. I personally prefer this mode, because here you are placing the crosshairs on the enemy, whereas with leading, you are trying to hit some imaginary spot in front of the target.
Eh, it's really hard to explain this. Just try it goddammit.
