In Deuteronomy the people were being prepared to go into the land. Remember in Exodus they are freed by God's mighty hand from the yoke of slavery to the Egyptians. The are led by God to Mt. Sinai where thy received the Law the first time, but then as they near the Promised Land they are shown to be faithless and refuse to go in and possess what God has promised to give them. Because of this they wander in the desert for 40 years, and during that time every male who was 18 or older at the time of the Exodus dies with the exception of Joshua and the other faithful spy.
Now we are at the end of that 40 years, and preparing to go in with the new generation to take the Promised Land, and one more time the Covenental relationship between God and them is being given and laid out, so again we have a reciting/expounding of the Law given to them.
Also of great importance in this book is the passing of leadership from Moses to Joshua as Moses will not enter the promised land until the life of Christ (remember the mount of transfiguration).
Hope that helps some, if you want me to expound or have more specific questions, feel free to ask away.