What does "Showing X of Y items" mean in an epic?

The "Showing X of Y items" text appears when the epic contains more backlog items than are currently shown inside the epic:

This is the case if some of the backlog items are placed separately from the epic in the backlog, or if you are currently filtering which backlog items to show. In the Releases, Sprints, and Kanban tabs, it is also the case when some of the backlog items in the epic are assigned to a release, sprint, or Kanban board.

To view all of the backlog items in the epic, open the popup dialog for the epic. For example, click on the name of the epic, or on the "Showing X of Y items" text.