I will explain Oma Falk 's answer.
Let list1 = [1,2,4]
And list2 = [2,4,6,8]
What set function does is that it creates a set but a set only contain unique entries, so all the duplicate entries are removed.
So this set will look like this
But it is no more a list, so to convert it back to list we use list()
So we can say that
list(set(list1+list2)) == [1,2,4,6,8]
If this is not what you want to know then explain your question.