This is a Good question and don't feel bad if your not sure what the cutout actually is. Not everyone knows what the term Cutout actually applies to in the world of grilling. This is contractor talk and now your asking how to speak contractor language to a sales person that is going to select the correct size for your grill.
Going into the store or online with these dimensions will save you time and frustrations.
See a simple sample diagram showing cutouts for Grills, Doors, and Drawers.
If your replacing an old or existing grill, you typically will have very few choices to match that cut out. If it fits just on the edge, that is ok, you do not need a great deal of support to keep the grill in place. Consult a contractor for the correct support but it is pretty much common sense.
Sometimes, that same cutout for your grill is no longer available, now matter what manufacture you turn to. If it is too big, you might get away with adding a grill liner, which can work but they are not always available.
If you cannot find a Grill or Grill liner to fill the cutout area, then you will have to consider maybe cutting out a larger area for a slightly larger grill. People are going bigger with Grill sizes and are more popular, just like they have been doing for years, with the size of your TV screens.
Depending on the structure, you can cut into it and maintain the support you need to not have to tear out the top and re do it.