Popcorn salt has a finer texture than normal table salt, which means it is better suited to coating every bit of popcorn with tasty seasoning. Making it at home is easy and quick, so if you aren't doing it -- or worse, spending money on flavored popcorn seasonings -- now is a good time to start. Image from pedrik.
Toss some table salt into a food processor and grind it until it has a fine texture, about 10 seconds. If you have a mortar and pestle, you can use that for small batches or single servings. You can also throw in other seasonings, like dried herbs, chilli flakes or garlic powder to give each handful of popcorn as much flavour as possible.
Hit the link below to see Alton Brown's full recipe for perfect homemade popcorn, salt included.
Darn Near Perfect Popcorn [Alton Brown]