Leading the target
When trying to hit a target which is angling across your field of view to some extent, you need to aim ahead of the target else it'll be gone by the time your shots arrive. In other words you need to aim at where it's going to be. This is called leading the target, and it's a valuable skill which you can develop with practice, enabling you to take down such targets more quickly. The slower the shot speed and the faster the target's speed across your field of view, the more you have to lead it, i.e. the further ahead you have to aim.
All plasma weapons are slow enough to require this technique on occasion - especially the plasma pistol with its relatively low shot speed. The rocket launcher is another such weapon. I quite enjoy wearing down Banshees and Ghosts with plasma weapons, and if you try that yourself, you'll soon be getting a lot of practice at target leading. But the technique is not just for use against vehicles of course; you can also need it against enemies on foot.
Applies to tagging too
Leading the target is also a technique which applies to tagging with plasma grenades. Ok you're not using a gun in this case, but you're sending out something to try and hit the enemy, which is all that matters. Most commonly it would be something you do when trying to tag an enemy vehicle (Ghost, Wraith, Banshee), but sometimes you might need it against a covie on foot - for example when he's running towards one or more Marines and you're off to the side.