The State Department is crucial to the development and implementation of America's foreign policy and international relations. Among it's most important functions is the advising of the President, Congress and the Cabinet as to how, why and when to negotiate and interact with other governments.
The State Department administers the US embassy sstem around the globe, coordinates ambassador posting and embassy staffing, international protocols, consulates, and often, direct negotiations with other countries. It is also essential in the US role in the United Nations and international law.
It is led by the Secretary of State, who is one of the President's most important foreign policy advisers, and who is most often responsible for peace, trade and treaty negotiations with foreign countries.
In the State Department itself are hundreds of "desks" assigned to the 194 other countries on Earth and staffed by language, historical and cultural experts on each of those nations. This allows a President to get instant information and advice on how to negotiate with foreign countries and what intelligence we have on those nations.