Accusative refers to the direct object. Nominative refers to the subject. Obviously the apple here is the direct object, not the subject. Sie is the subject. So the apple is in the accusative case.
Just google "German cases" and you'll find plenty of material explaining the difference, it'll take time but at some point things like this just start making sense when learning a language. I wouldn't use Duolingo as the ONLY learning method. You need to utilize all the tools you can find :)
There is a button at the top of the page (not the duolingo bar, below it) that says "Tips and Hints". If you click it, it will show you a box containin the things you are learning on the level.
Accusative is, as Jokor already said, when something is being done to it. You can easily tell it's not nominative, because only the subject of the sentence (whatever is doing the action; here: "They") is in nominative.
How can I find the clear difference between both cases of the translation :
- They are eating the apple
- You (formal) are eating the apple
in German the translation is " Sie essen den Apfel", how come?
Den is normally the, but it can be, depending on context, also this or that. But never a/an. That would be with a masculine word in the accusative case: einen.