Armstrong's advice is sound. A gamelogic is just an invisible unit. It can do pretty much anything a unit can do: follow waypoints, ride in trucks (handy for when you want the seat next to the driver to be empty, but you want some loons in the back), all kinds of things.
For many scripts, you put nothing at all between the square brackets []. When you do, it is specific to that script. It could be a unit, or several units, or markers or whatever.