This is the law that Moses set before the people of Israel.
The events in the book of Deuteronomy take place immediately before the Israelites enter into the promised land.
Moses is effectively restating the events which have led to this point, including a restatement of the law and the promises which God has made for Israel's future.
Time and again in the bible we see the law of God re-read and restated to the people of God. The law is another name for the words which God has revealed in the bible. From this repeated example in scripture, we can learn that we should be continually reading, re-reading, and thinking about the words from God in the bible, and how our lives should be shaped to follow these words.