Grill seasoning is like heroin.
It is the magic ingredient (among others) used mainly by fast food restaurants to make their food appetizing. Other restaurants also use variants on the theme.
At its most basic, it is salt and pepper. Salt brings out the flavor in food, and stimulates the taste buds. Pepper adds some zing, some "sophistication" to the dish. A little garlic goes a long way, and ginger makes everything go better.
A simple burger fried in a pan over the stove can be a little bland, compared to Mickey D's offerings of ankles and ears, but shake a little homemade grill seasoning on at least one side, and voila, a tasty treat!
Shake it on potatoes about to be baked after washing, to give the skin some extra oomph. Shake it on anything that isn't already seasoned carefully in some other way.
To do this at home, you'll want to get a nice shaker, a can with a bunch of small holes in the lid, with a nice plastic cover.
A basic mix is mostly salt and pepper (probably 2:1), with a generous helping of garlic powder and a hint of ginger added. A bit of onion powder can go a long way, too, as this recipe's fossil record will show. Add any other seasoning that will make it through the shaker holes to taste.