Light often symbolizes clarity, hope, and purity. From this metaphor, therefore, we might infer that "great" people are hopeful and pure and have the capacity to see or think clearly. In the fourth line of the first stanza, the speaker indicates that the greatness of "those who were truly great" is "Endless."
I Think it Tells About.. the legacy that great people left in the world during their lifetime that made a mark since soul's history ( or the beginning of time) that serves as a Model for us to follow and improve ourselves.
-( its the Legacy that they left...)
The distiinct quality of those who were truly great is having great ambitions and since the very beginning until the time he/she was born.It's him who never stops dreaming and believing that is truly great.