The preamble to the Constitution is an introduction to the document. You can think of it like a book summary on the back of a book that shows you what you should expect from reading the full book.
The preamble states the purpose of the Constitution and the intention of the entire document full of laws. The preamble is not actually a law or a legal document, but it is used to make it clear what to expect in the Constitution. Namely, the preamble says that the Constitution aims to create laws around justice, peace, defense, welfare, liberty, and prosperity for a "more perfect" country for Americans.
We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defense, 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.
Although the preamble is not a source of power for any department of the Federal Government, 1 the Supreme Court has often referred to it as evidence of the origin, scope, and purpose of the Constitution. 2
''Its true office,'' wrote Joseph Story in his COMMENTARIES, ''is to expound the nature and extent and application of the powers actually conferred by the Constitution, and not substantively to create them.
For example, the preamble declares one object to be, 'to provide for the common defense.' No one can doubt that this does not enlarge the powers of Congress to pass any measures which they deem useful for the common defense.
But suppose the terms of a given power admit of two constructions, the one more restrictive, the other more liberal, and each of them is consistent with the words, but is, and ought to be, governed by the intent of the power; if one could promote and the other defeat the common defense, ought not the former, upon the soundest principles of interpretation, to be adopted?'' 3
2 E.g., the Court has read the preamble as bearing witness to the fact that the Constitution emanated from the people and was not the act of sovereign and independent States, McCulloch v. Maryland, 17 U.S. (4 Wheat.) 316, 403 (1819) Chisholm v. Georgia, 2 U.S. (2 Dall.) 419, 471 (1793); Martin v. Hunter's Lessee, 14 U.S. (1 Wheat.) 304, 324 (1816), and that it was made for, and is binding only in, the United States of America. Downes v. Bidwell, 182 U.S. 244, 251 (1901); In re Ross, 140 U.S. 453, 464 (1891).
3 1 J. Story, Commentaries on the Constitution of the United States (Boston: 1833), 462. For a lengthy exegesis of the preamble phrase by phrase, see M. Adler & W. Gorman, The American Testament (New York: 1975), 63-118.