Final answer:
Jefferson was referring to the power of judicial review, which is the authority of the Supreme Court to interpret laws and declare them unconstitutional if necessary.
Step-by-step explanation:
Thomas Jefferson was referring to the power of judicial review in the quoted statement. Judicial review is the authority of the Supreme Court to declare what the law is and to assess the constitutionality of legislative acts or actions by the executive. This power was established in the landmark case, Marbury v. Madison (1803), even though it wasn't expressly provided for in the Constitution. Jefferson's concerns highlight the potential of this power to influence and possibly undermine the foundations of the constitutional framework. Despite Alexander Hamilton's assurances in Federalist No. 78 that the judiciary would be the least dangerous branch because it had neither force nor will but merely judgment, Jefferson feared the unchecked power to interpret laws could be wielded to alter the balance of power within the government.