Féconde translates to "fertile" or "fruitful". While often used in a biological sense, it's used here metaphorically to describe a rich and abundant joy.
In the lyrics, "la joie féconde" (the fruitful joy) suggests a joy that is not only deep but also generative, producing more happiness and positivity. It's a beautiful and less common adjective that adds depth to the song's message of hope and shared happiness.
"Il y aura" ("There will be") is Claudio Capéo picturing a life still ahead with someone he loves. He starts small and physical: a reflection, an embrace by the Seine, suitcases finally set down after years of waiting. It's the relief of two people choosing to actually start their life together.
From there the song widens out. There will be friends, laughter, children playing in a place they've built together. He doesn't pretend it will all be easy — he names the pain, the anger, the noisy arguments too — but the answer he keeps coming back to is simple: say "I love you" anyway, and hope anyway.
The last verse is the heart of it: when the future looks dark, we tell each other stories to give one another hope. It's a song about choosing togetherness and hope as a daily decision, not just a feeling.