The Preamble:
We the people of the United States of America, in order to form a more perfect union, establish justice, ensure domestic tranquility, provide for the common defense, promote the genreal welfare and secure the blessings of liberty to ourselves and our posterity, do ordain and establish this constitution for the United States of America.
It states that they're going to provide freedom for the nation, provide support for our soldiers (I think that's what they mean by common defense), enforce laws, and basically make the nation just a peaceful, free land.
Let me know if this helps.