The correct answer is introduce and explain the core ideas of the new government.
The preamble discusses how this new constitution was developed in order to form a "more perfect union." Along with this, the preamble also discusses how this new system of government will protect the rights of citizens, provide for a common defense (aka military), and to ensure peace. This preamble (introduction) provides a brief background of what rights/laws were implemented in the body of the US Constitution.