Foreign policy refers to the political decisions that a country makes when it comes to dealing with other nations. "Foreign" refers to nations abroad and "policy" refers to political decisions.
In this case, the United States witnessed an important transformation in foreign policy during the late 1800s. Since Independence, the United States had wanted to stay out of conflicts in the international arena, and was, therefore, quite isolated. However, during the 1800s, this approach changed, and the United States wanted to increased its contact with other nations, often through an imperialistic approach. The United States wanted to increase trade with other nations, and in some cases, expand its control over their territory.
This became even more clear during the World Wars, as the United States decided to participate in both of them.