The English word 'epic' means 1: constituting or having to do with or suggestive of a literary epic; 2: a long narrative poem telling of a hero's deeds; 3: very imposing or impressive; surpassing...
An epic song would be a song based on something important, outstanding, or legendary. An example would include "The Wreck of The Edmund Fitzgerald". It is basically an intense poem written to music.
An epic is a legendary story, usually set in the distant past, of heroes who are more than usually powerful. They often are mixed with accounts of gods or other deities. Not infrequently they are...