The poem starts out sounding like an homage to great people in history. As the first stanza develops, however, the speaker seems to be saying more about how greatness is born of the connection between generations and the connection between humanity and nature.
The great ones remember the soul's history, meaning they understand the past. They have a significant, maybe even spiritual, connection to human history and world history. The "endless and singing" "corridors of light" represent the perpetual enlightenment that is always available for human potential. The energy of the world and the universe runs through that greatness. Everything is connected.
In the second stanza, the speaker entwines notions of human life and the natural world. This suggests an inherent connection. There is something ecological or symbiotic about this. Note that in this stanza, the speaker warns:
Never to allow gradually the traffic to smother
With noise and fog the flowering of the spirit.
The speaker is suggesting that "traffic" (technological noise and distraction) can disrupt the human/nature connection. Spender uses the "flowering of the spirit" metaphor to emphasize this connection between nature ("flower") and the human "spirit."
In the final stanza, the speaker personifies aspects of nature, claiming that they praise (fete) the great ones in history. In the final two lines, the speaker concludes by describing how the influence of great people is left in the "vivid air." Their influence can be seen and remembered in human culture. Spender ends with another notion of nature ("air"). It's as if to say their influence is not only written in books, but also in the natural air itself.
Humans are of the earth. Their energy comes from the Sun. Therefore, their greatness is literally and poetically linked to nature. Connection is an important theme here. The greatest ones remember the soul's past. Spender is showing how they are connected to others across time and how they are connected to nature itself. It is a very optimistic and hopeful poem because it is all about human potential as well as historical, ecological, and spiritual connection.