It is based on Exodus 19.  This story recounts Israel’s first experience as soon as it got into the wilderness after being rescued from Egypt.  Moses makes several trips up and down the mountain to communicate with God.  God wants the people to prepare for a theophany, in which God will come in a thick cloud, accompanied by smoke, fire, thunder, and other phenomena.  Moses is instructed to prepare the people for this by consecrating them.  There are requirements to be observed, since getting close to a holy God can be dangerous.  Eventually, God comes to the mountain and speaks to the people directly.  This is one of the few times that God does not speak through a mediator.  What he says is the ten words, or ten commandments.  This theophany emphasizes the significance of those commandments.