When we talk about things, we sometimes need to say who or what did what to whom or what. This is called the accusative case.
In English, we don't always change the words to show that something is in the accusative case. For example, "I see a bee" and "the bee sees me" have the same words, even though "a bee" and "me" are in the accusative case.
But in other languages, like German, we have to change the words to show that they are in the accusative case.
So, imagine you have a toy truck and you want to say that you are playing with it. In German, you would say "Ich spiele mit dem Lastwagen." The word "Lastwagen" means "truck" and it is in the accusative case. The word "dem" tells us that.
But if you wanted to say that the truck was playing with you (which would be a very silly thing!), you would say "Der Lastwagen spielt mit mir." Here, the word "mir" is in the accusative case, and tells us that you are the one being played with.
So, the accusative case is just a way to show who or what is being affected by the action. It helps us understand who is doing what to whom or what.