Nominative-accusative is just a way that some languages, like English or Spanish, use different words to show the difference between the subject of a sentence (the person or thing doing an action) and the direct object (the person or thing being acted upon).
When we use the nominative case, we are talking about the subject, the person or thing doing the action. It's like saying "I" or "he" or "she" or "they" when we are talking about who is doing something.
The accusative case, on the other hand, is used to show the direct object, the person or thing that is being acted upon. It's like saying "me" or "him" or "her" or "them" when we are talking about who is being affected by an action.
So, in English we say "I kicked the ball" or "he ate the cake". In the first sentence, "I" is the subject and "the ball" is the direct object. In the second sentence, "he" is the subject and "the cake" is the direct object.
Other languages, like German or Russian, also use the nominative-accusative system to show the subject and direct object in a sentence.