In America, there is always talk about what the Constitution says and what it does not say. People argue over whether it should be obeyed to the letter or whether it is a document that, like the bible, is a left up to interpretation. It takes some effort to examine the Constitution objectively and what it actually means, both historically and in a legal context. Of course, it starts with the preamble.
(Find the text of the Constitution here.)
One of the things that stands out the most about the preamble to the U.S. Constitution is the term "We the People." To understand this, one has to understand what "the people" stood for in this context. Well, it was referring to the people who were writing and voting for the document. This means that, while it is a very forward statement for the time, it still did not include slaves, most women and most black people. Note that many free slaves were allowed to vote, so the earliest vote in the United States had blacks voting.
Now, before the Constitution starts to look positively barbaric, it is important to remember that, even with its exclusions, this term "We the People" meant something that was nearly unheard of. It meant people who didn't necessarily own land and a very widespread group of them was voting on a single document that was essentially creating a government. Even the earliest democracies had more caveats than this far-reaching statement, so there is that. Remember, democracy was very limited in the world at that time.
The other most important part of the preamble of the U.S. Constitution is its reasoning for itself. It was all about providing defense, liberty, domestic tranquility and justice for the people of the United States through a common, "more perfect" union. Reading on, it is easy to see how the framers of the Constitution proposed to succeed in these lofty endeavors. In some areas, people felt it fell short straight off, but it has remained a strong document for more than 200 years.