Read the preamble to the constitution.

we the people of the united states, in order to form a more perfect union, establish justice, insure domestic tranquility, provide for the common defence, promote the general welfare, and secure the blessings of liberty to ourselves and our posterity, do ordain and establish this constitution for the united states of america.

according to the preamble, the phrase “a more perfect union” refers to

improving upon a previous attempt at creating a new union.
duplicating the original government system set up by the union.
creating a union with stronger ties between individual states.
building up the union with of foreign governments.