To what is FDR referring to when he says that the democracies, by fighting aggression, "are keeping war away from our hemisphere"?

This quote comes from the now famous Four Freedoms speech, delivered by Roosevelt as the State of the Union address on January 6, 1941. The international context was very negative. Hitler and his Axis allies were winning in Europe. Britain was greatly menaced and the Japanese were gearing up for all-out war in the Pacific and in Asia.

Roosevelt, aware that scientific progress had eliminated the physical advantage of the USA being separated from Europe and Asia by such vast expanses of water had been arming and funding the British for years and had also increased military spending and the Navy in order to keep Japan at bay.

He knew that once Hitler had conquered Europe and Japan had conquered the East, all the populations and resources of these vast reasons would be used to directly attack the USA, which would eventually find itself in numeric inferiority so in order to prevent such a dark future he chose to send weapons, supplies and money to the British.