Preamble to the United States Constitution
In general, this part of the Preamble means that one of the new government's jobs would be to make things better for the whole country.  Specifically, this part of the Preamble has to do with the government's powers to tax people and spend money.  It means that the new government will only have the power to tax and spend on things that are good for the entire country. The government will not be allowed to spend money on things that will be good for only a part of the country, or only some people in the country.