A-B means that elements which are elements of A but not B.
We will look at an example of the set difference. To see how the difference of two sets forms a new set, let's consider the sets A = {1, 2, 3, 4, 5} and B = {3, 4, 5, 6, 7, 8}. To find the difference A - B of these two sets, we begin by writing all of the elements of A, and then take away every element of A that is also an element of B. Since A shares the elements 3, 4 and 5 with B, this gives us the set difference A - B = {1, 2}.