Complex sentence: The duty of fair representation is the obligation, incumbent upon U.S. labor unions that are the exclusive bargaining representative of workers in a particular group, to represent all those employees fairly, in good faith, and without discrimination.
Split into simpler sentences: The duty of fair representation is incumbent upon U.S. labor unions that are the exclusive bargaining representative of workers in a particular group. It is the obligation to represent all employees fairly, in good faith, and without discrimination.