The United States was not the first country to have a "constitution." People in England, the mother country of the American colonies, also used that term to describe the rules structuring their government. But to the British, the constitution was not a single document or code. It was not something they could print on expensive paper or preserve in a museum. It was simply all the various laws and customs that determined how their government was "constituted" — how it normally behaved.
Americans of the Revolutionary War era took this concept to a new level. Many saw the relationship between rulers and ruled as a contract, one that gave citizens responsibilities but also placed obligations on their leaders. There were things the government was permitted to do, and other things that were forbidden. But how would leaders know their privileges and their limits without some kind of guidance? How could citizens know whether their government was violating the bargain, unless a list of Do's and Don'ts existed somewhere?
The Constitution of the United States evolved to fulfill this desire for a binding contract. It enumerated (listed) the powers available to the national government and explicitly forbid other actions (a list that grew with the first several amendments). The U.S. Constitution therefore defined the basic structure of national government, but it did so in a fundamentally new way — because it was a written code that the government lacked authority to change. For this reason, the U.S. Constitution is considered the oldest working constitution in the world.