The main theme of the poem centers on the necessity of remembering, celebrating, and cherishing the "truly great" among us. The narrator doesn't specifically state who these "truly great" people are, but they might be famous writers and poets the narrator knows.
The narrator praises these "truly great" individuals with highly laudatory words. During his life, Stephen Spender was infatuated with the idea of fame and greatness. He yearned for success and dreamed about it; as a rule, he thoroughly enjoyed being surrounded by successful writers, authors, and poets. It's no surprise, then, that he lavishes high praise on the "truly great."
He likens these individuals to essentially superior beings, born from divinity:
What is precious, is never to forget
The essential delight of the blood drawn from ageless springs
Breaking through rocks in worlds before our earth
Here, the "blood" of the "truly great" is of divine origin. It is "drawn from ageless springs," meaning God himself has infused his divine essence into the bloodstreams of the "truly great." So, "ageless springs" are a metaphor describing God or Providence, whose power is ageless and eternal. This "ageless spring" is formidable enough to break through "rocks in worlds before our earth." Essentially, the narrator may be intimating that the "truly great" have charted new territory in areas beyond the average citizen's comprehension. Because of this, we should remember them with deep gratitude and strive to emulate their wisdom and nobility of spirit.
Besides using metaphors to reinforce his points, Spender also uses synecdoche to warn us against letting our busy lives crowd out what's truly important:
Never to allow gradually the traffic to smother
With noise and fog, the flowering of the spirit.
In the above, Spender mentions the "traffic" that smothers "the flowering of the spirit." Traffic is part and parcel of bustling, modern lifestyles. Traffic, along with grueling work schedules and demanding familial responsibilities, often robs us of time to contemplate the achievements of the "truly great" and emulate them.