When you have an appetite, or a good appetite, it means you feel hungry and ready to eat. If you don't have an appetite, it means you're not hungry--you could be feeling sick, or maybe you're just still full from your last meal. If something makes you lose your appetite, it's grossed you out so much, or made you so sad or angry, that you don't want to eat anymore.
Ron has a big appetite as he sits down to dinner:
When you exercise for a long time, or play hard or study hard for a long time, you work up an appetite. That means you get more and more hungry and ready for food. If you have an appetite for some specific food, then you want only that one. I've always got an appetite for mac and cheese.
You can have an appetite for something that isn't food. You could have an appetite for adventure, meaning you're "hungry" for adventure: you crave it, you want it, and you can't wait to get it. You could have an appetite for books, meaning you're "hungry" to read: you crave books, and you can't wait to read them. Lots of them!