What is the inverted output for on a D flip flop?
Very fundamental thing. Flip-flops are composed of logic circuits that have cross coupled feedback such that they "hold" the last established state. These bi-stable circuits are often composed of inverting gate elements that invert twice around their cross coupled feedback to realize the latched state.

Simply the Q output is the output of one side of the bi-stable circuit. The Q' is the other output which, in the case of cases where inverted logic elements are used, ends up being the logical inversion of the Q output. 

So if Q = 1 the Q' = 0. Likewise of Q = 0 the Q' = 1.