Epic are used in Agile product management to structure tasks and establish hierarchy during the Agile Development phase. A user story is a smaller task that is part of an epic, which is a larger piece of work. An Epic frequently spans several sprints, teams, and even numerous projects. Before they can begin to extract functionality from epics, product folks must first deconstruct them into stories.
Within an Epic, all user stories have the same strategic objective i.e. Agile Development and high-level client objective. Similar to this, when numerous epics have a similar goal, they are compiled into a larger body of work known as a "theme." The scope of an Epic will alter as sprints are finished and our knowledge of the demands of the client grows, and new user stories may be added to or taken out of the epic.