Imperialism refers to when a country controls another country politically and economically, sometimes against its will. The United States became imperialistic in the late 1800s.
We manufactured a war against Spain in Cuba in 1898. American newspapers exaggerated the poor treatment of the Cubans by the Spanish. American public opinion turned against Spain. After the USS Maine exploded off the coast of Cuba, we declared war on Spain. As a result of the Spanish-American War, we gained controlled over Guam, Puerto Rico, and the Philippines. The people of the Philippines wanted their independence and weren’t pleased that we were controlling them.
The United States gained influence in Panama by helping Panama revolt against Colombia. We wanted to build a canal in the region of Colombia that is now the country of Panama. When Colombia refused our offer to build a canal, we encouraged and supported a revolution that led to Panama becoming independent. We offered Panama the same deal we offered Colombia, and they accepted it. We then built the Panama Canal and controlled it for the rest of the 20th century.
These are two examples of U.S. imperialism.