This ability is described in full on the Army Rules page of Codex: Space Marines.
Designer’s Note: The rules presented in this section assume that the ADEPTUS ASTARTES units in your army are from the Blood Angels Chapter, but they can also be used to represent any Blood Angels successor Chapter, such as one described in the background section of this book, or even one of your own invention. However, players who wish to faithfully recreate the Blood Angels Chapter on the tabletop should only include BLOOD ANGELS EPIC HEROES if their collection is intended to represent the First Founding Chapter itself; Mephiston is the Chief Librarian of the Blood Angels, for example, and not of any of their successors.