He simply mean to make your first level empty, but working with the core actors (like the hero and exit door actors) and the scene size/background color, etc.
Then in GameSalad you copy and paste your scene so you don't have to do those all over again for every levels.
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The actor that ends the level (ex: an exit door) will change the attribute to unlock the level. You can just make this at the end.
I personally think its best to get level one working flawlessly and then copy it and modify the layout for level 2,3,4,5,6,…….Etc.
Then in GameSalad you copy and paste your scene so you don't have to do those all over again for every levels.