Please explain the Roosevelt Corollary to the Monroe Doctrine.
The Monroe Doctrine said that European countries should essentially stay out of the affairs of countries in the Americas. The US did not want European countries coming and trying to recolonize any of the free nations of the Americas. The Roosevelt Corollary, issued almost 100 years later, expanded upon this in a way that gave the US more power to intervene in the affairs of countries in the Americas.
The Roosevelt Corollary said that the US had the right to intervene whenever an American country became so badly managed that the Europeans were likely to want to intervene. In other words, if an American country was about to default on its debts (for example) the US could come in and take over the running of that country so that no European country came in and did so. This gave the US the power to intervene in the affairs of other American countries more or less whenever it saw fit.