Answer:
Technically, it serves as an introduction. It assigns no powers to the government. But to break it down, it would be like this:
Form a more perfect union= fix the problems of the Article of Confederation
Establish justice= have courts
Ensure domestic tranquility= keep the peace in the country through law
Provide for the common defense= defend the country
Promote general welfare= self-explanatory
Secure the blessings of liberty to ourselves and our posterity= equality to all