"we the people" are the first words of the preamble. what do these words mean and how are they reflected in the united states government system today?

When the Constitution of the United States begins with the words, "We the people," it is asserting that the power to organize a government is vested in the people of the country that is to be governed.  This was an idea that the American founding fathers took from Enlightenment philosophers such as John Locke.  Locke had debunked the theories of kings ruling by divine right, in his First Treatise on Civil Government.  Locke's Second Treatise on Civil Government set forth theories on how a social contract is established that invests the government with its power and limits its power by the rule of the majority through legislatures.

We still follow that pattern in US government today, with the people electing representatives at local, state, and federal levels. When the Constitution was written, not all citizens of the US held voting rights.  Women and slaves were excluded, and a certain measure of property ownership was required for voting rights by white men.  Today, all citizens of legal age have equal rights as voters.  So in today's America, "We, the people" actually means all the people.