What is the purpose of the preamble of our constitution

to promote general welfare

to form a more perfect union

establish justice

inure domestic tranquility

secure our blessings of liberty and of our prosperity

The Preamble is essentially the thesis statement of the Constitution; it outlines the purpose of the text ("we the people ... do ordain and establish this Constitution"), what will be discussed in the text ("provide for the common defense, promote the general welfare"), and the overall message the authors are trying to get across ("in order to form a more perfect union").