The Preamble states who is creating the Constitution and why. It grants no powers and creates no rights on its own, courts treat it as a statement of purpose, but its opening words, "We the People," announce the document's central idea: the government's authority comes from the people themselves, not from the states or a monarch.
We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.
One sentence, six goals: a stronger union, fair courts, peace at home, national defense, the general well-being of the country, and liberty preserved for future generations. The framers wrote it in the people's voice deliberately, the Constitution's authority rests on ratification by the people's conventions, not on an agreement among state governments.