The Constitution is called the "highest law in the land" primarily because all other laws in the United States must conform to the principles and provisions established within it. This concept is rooted in the Supremacy Clause, found in Article VI, Clause 2 of the Constitution, which states that the Constitution, along with federal laws and treaties made under its authority, shall be the supreme law of the land. This means that if there is any conflict between federal law and state law, federal law prevails.
While the Constitution is not the oldest document in the United States, it serves as the foundational legal framework for the country and establishes the authority of government, the rights of individuals, and the principles of justice and governance. Therefore, your second statement is correct: all other laws in the United States must follow the rules laid out in the Constitution.