Splendore translates to "splendor" or "radiance". It's a beautiful and evocative word that isn't as common in everyday conversation as words like "amore" or "felicità".
In the context of "La Valle Dell'Eden," it refers to the magnificent beauty and glory one will experience in the metaphorical Garden of Eden, a place of peace and love. The lyrics promise, "Certamente lo splendore lo vedrai" (Certainly you will see the splendor), making it a compelling and uplifting word to feature.
La Valle Dell'Eden turns the idea of paradise into somewhere you can reach today. Eden here is not a place on a map. It is what appears the moment we find happiness again, and the singer insists you can see its light in ogni strada, in ogni città (in every street, in every city). The condition he keeps coming back to is love: se hai l'amore là dentro di te (if you have love inside you), you find the valley of Eden, a country dove notte non c'è (where there is no night), and one that is troppo lontano non è (not too far away).
The middle verse is where the song gets honest. In a world where nothing seems worth much anymore, even laughing can drag you down. His answer is not to escape or to travel somewhere better, but to turn and look around, and inward: ritroverai la pace dentro di te (you will find peace within yourself).
By the last verse the message is plain. When you stop chasing and start looking for la semplicità (simplicity), you arrive, and the Eden you find turns out to be your own. Sung over and over, sì, nella valle dell'Eden sarai sounds less like a promise about the future and more like an invitation to notice where you already are.