Simplified
The Preamble introduces the Constitution and lays out the fundamental purposes and guiding principles of the government: to form a more perfect union, establish justice, ensure domestic tranquility, provide for the common defense, promote the general welfare, and secure liberty for present and future generations.
Original
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.