This is the same thing as A minus A. This is literally saying let's take set A and then take all of the things that are in set A out of it. I start with a 5, but there's a 5 in set A so I have to take the 5 out. There's a 3, but there's a 3 in set A so I have to take a 3 out. I'm going to take all of these things out. I'm just going to be left with the empty set, often called the null set. Sometimes the notation for that will look like this.