It is the elements which are not present in a sent, Kinda' like its opposite. For example:
U={ A,L,G,E,B,R,A}
A={ L,B,A,}
B={ G,E,R }
What is the B' ?
B' means complement of B . It is represented by the ' at the top of the set.

B' = { L,B,A }
You are not supposed to double the A just because it is there in the set. Because in the end, it is just the same letter.