With regard to "objective" v. "subjective", I think a couple of examples might be helpful.
If we ask, "Which President was better -- Washington or Lincoln?" there is no objectively correct answer. It's all a matter of individual opinion. There's no way to verify that one answer is right and the other is wrong.
If we ask, "How many amendments to the United States Constitution have been ratified by at least 3/4 of the States?" there is a correct answer (27) that doesn't depend on individual opinions and a way to verify that the answer is correct and that all contrary answers are wrong.
Suppose we ask, "Is rape morally wrong?" Most of us would say yes but not every in history has agreed (Saddam Hussein and his sons, for example). Is this like the Presidents question -- a matter of personal opinion with no right answer? Or is it like the amendments question -- with a correct answer that doesn't depend on individual opinions and a way to verify that the answer is correct and the opposite answer is wrong?
Craig's argument is that if there is no final judge on moral issues (God), then all moral statements are relative, like the Presidents question. We could give arguments and reasons for our opinion, but in the end there would be no way to verify that our arguments and reasons were right.
With regard to the question of whether God's moral judgments are based on whim, C.S. Lewis argued that this is not so, that God's moral judgments are a part of His character, that rape is wrong not because God arbitrarily decided so but because it violates the character and nature of God, and the only way we can know the character and nature of God is for Him to reveal it to us.