Let's talk about the accusative case in German. The accusative case contrasts with the nominative case in that now we are not talking about the subject of a sentence anymore, but of its object.
Consider this sentence:
In this very basic sentence, somebody (Der Junge) does something (throwing the ball). Whoever does the action is the subject of the sentence and whatever object the action is done to is the object of the sentence. Another way to put it could be to say that what or whoever is the receiver of the action is the object of the sentence.
So, in this sentence, the ball is the object because it is being thrown.
Now, in English, this almost doesn't matter: You can say "The boy throws the ball" or "The ball throws the boy". In both cases, it's the boy and the ball - it's just the position in the sentence that changes.
In German, this is very different. In German, nouns change their form depending on what role they play in a sentence. When the ball is the originator of the action, it will be in the nominative case, whereas it will be in the accusative case as the receiver of the action.
This is why it's not "
Der Junge wirft der Ball." but "Der Junge wirft den Ball."