The accusative case is a case which we use to point out the direct object of a sentence. Sounds confusing, but actually it’s dead easy: it’s the case we use for the thing in the sentence which isn’t doing the verb but is having the verb done to it. For example:
I dropped the cheesecake on the floor. – Here, ‘cheesecake’ would be in the accusative because it’s the thing being dropped.
Ich schmeiße meinen Laptop aus dem Fenster. – The laptop is in the accusative case here, because it’s the thing being chucked out the window.
Here are a couple of useful tables to show you how the articles change in the accusative case. As you can see, the articles only change in the accusative case for masculine articles, because the masculine gender is weak. Quite right.
The declensions of the definite article
The declensions of the indefinite article