Epics represent large pieces of work like major features in a software or an initiative in a project. Epics can also be used to group similar user stories.
For example, if the Facebook team was using Freshrelease, then they would track News Feed as an epic and other features inside news feed like posting on news feed, liking posts, commenting on posts can be tracked as user stories.
In agile, Epics cannot be added to sprints directly because usually the time taken to complete an epic will be more than the sprint duration. So, the epics need to be broken down into multiple user stories, which can be moved to sprints.