so called because it comes up in Plato's dialogue Euthyphro. To paraphrase to fit the god of the Pentateuch/Old Testament:
"Are the 10 Commandments true because God decreed them, or did God decree them because they were true?"
The first option is called Divine Command Theory and it has been rejected by great theologians and philosophers from Plato through St. Thomas Aquinas, Leibniz and onwards. It has several problems, the most serious being:
(a) It makes God's commandments completely arbitrary. He could have said "Thou Shalt Rape and Pillage" (well, he pretty much does elsewhere in the OT, but that's besides the point) and then THAT would've been morally right. But we don't believe that. (If you say "But God wouldn't have commanded that" then explain why not. It can't be that rape and pillaging are antecedently wrong, because that's the SECOND option, not divine command theory.)
(b) It makes God completely whimsical. If you ask "Why did God pick THOSE commandments" the answer has to be: for NO REASON AT ALL. Because if there was a reason, then it would imply that things are good or bad before God decrees them, which, again, is the other option.
(c) It means the statement "God is good" is meaningless, because there is no standard of goodness outside of God by which we can assess him/her/it. It would be like saying "God is God" Well, duh. And so what?