What does in order to form a more perfect union mean?
"In order to form a more perfect union" means that the government was not perfect but that our founding fathers had a goal.
The word "union" refers to the union of the different colonies or states. Their "union" did not start out perfect, so the founding fathers set out with the goal of creating one, a union between all the colonies (or states) so that the common good and defense is provided for and domestic tranquility and prosperity could follow afterward.
Alone, each of the colonies was vulnerable to attack. They could not provide for their defense and ensure their long-term independence by themselves alone so they formed a union between them, so that they could win their independence and protect themselves against the British Government and the French.
There is a small community that believes that this part of the preamble is a part of a masonic code.