I assume A, B and C are sets.
Think of the sets as boxes, and take copies of all the elements (contents) of sets A and C, including duplicates, and put them into a box we’ll call set X. Take a look at what’s in set B and match with what’s in X. Another empty set we’ll call Y is going to be filled with copies of elements of B which are not in X. After going through the whole of B we will have filled Y with copies of all the elements of B which didn’t match elements in X.
We write all the elements of Y between curly brackets, and that’s the answer on the right hand side of the equals.