A "preamble" is an introduction to something - it sets the stage or gives the reasons for the writing of the document it introduces. The Preamble to the U. S. Constitution explains why the Constitution itself was written. It starts by asserting that the Constitution had been written by all the residents of the new country - "We the people of the United States".
The Preamble gives six reasons for the writing of the Constitution.
1. "in order to form a more perfect union" - The United States had tried to form a new government under the Articles of Confederation, but that structure had proven itself unworkable. There needed to be a stronger central government that had the authority to coordinate joint actions involving all the states on behalf of the country as a whole.
2. "establish justice" - The Constitution required establishment of a Supreme Court for the entire country and a system of lower courts to determine the legality of the laws being enacted in the new country.
3. "insure domestic tranquility" - The Constitution would support better relationships between different states. These safeguards would allow the citizens to feel safe and secure, living peacefully in their homes and working at their employment without threats of interference by neighboring states.
4. "provide for the common defense" - The Constitution authorized the newly created centralized government to create, train, and levy taxes to pay for an army and a navy. Military forces under the Articles of Confederation were sponsored by individual states; there was no way to make them work together to protect the country as a whole.
5. "promote the general welfare" - The Constitution provided a structure for a government that would allow individual citizens to have representation in the governing of the country. Representative democracy was seen as being the best form of government because it allowed the citizens to have input into the process.
6. "and secure the blessings of liberty to ourselves and our posterity" - The Constitution was enacted with the promise that the Bill of Rights would be ratified. Those amendments guaranteed individual rights and freedoms that were to be given to every resident of the United States when the Constitution went into effect and for as long as it lasted into the future, thereby including the descendents of "the people" who were writing the Constitution.
Having set out the reasons for writing the document, the writers concluded that "we the people...do ordain and establish this Constitution of the United States of America."