What was the Monroe Doctrine?
The Monroe Doctrine stated that the United States would oppose any further attempts by Europe to colonize lands in the Americas (western hemisphere), freeing up these lands for American influence.
The Monroe Doctrine was an arm of American foreign policy dating from 1823, issued under President James Monroe. It announced that the United States would consider any further efforts by European countries to colonize lands in the Americas (western hemisphere) as an act of aggression against the Unites States, requiring military intervention.
The Monroe Doctrine also stated that the United States would not interfere in the internal affairs of any existing European colonies in the Americas, nor would the U.S. get involved in the affairs of European countries (like the Greek fight for independence from the Ottoman Empire, for example).
In a nutshell, that was meant as a warning to the European countries: the US won't mess with anything you already have established here, but don't try to make any more colonies -- the Americas belong to us. The Monroe Doctrine's objective was to free the newly independent colonies of Latin America from European intervention, and avoid situations which could make the "New World" a battleground for the "Old World" powers -- while allowing the US to exert influence in the Americas undisturbed.
This move was politically and militarily motivated, certainly, but was also about economics and commerce; the US wanted to increase its trade with the lands of the Americas without stiff competition from European economies.
The U.S. lacked credible naval power at the time, so the Monroe Doctrine wasn't taken seriously at first by most European powers; however, Great Britain kind of helped to enforce the Doctrine with its naval power.