Gedeih'n is a contracted form of gedeihen, which means 'to thrive', 'flourish', or 'prosper'. It's a beautiful and somewhat less common verb that evokes growth and well-being.
In the song, it's used in the line "Mit dir daran gedeihen" (With you, it will thrive/flourish), referring to the growth of a child and the love surrounding it. This word captures the essence of nurturing and positive development, making it very evocative and memorable.
This is a song from a parent to a child, and In Extremo, a band better known for bagpipes and medieval stomp, sing it with their armour off. It opens like a little farming proverb: "Wo zwei Menschen innig lieben, fällt die Saat auf satten Boden" (where two people love deeply, the seed falls on rich soil). The singer admits he thought he was wise and "hab es einfach nicht gewusst" (simply didn't know it), and then lands on a neat image: for life you need "beide Gleise" (both tracks), "Vaters Kraft und Mutters Brust" (father's strength and mother's breast).
The chorus is pure reassurance. "Es wird schon werden" (it'll be fine) is an everyday German phrase worth stealing, and "Du bist mein Glück auf Erden" (you are my happiness on earth) gives the song its title. Then it turns heavier: "Darum gehe ich vor dir" (that's why I go before you) means the father expects to die first, and later says "Dich will ich nicht sterben sehen" (I don't want to see you die). In between sit the small, tender details, like "Die kleine Hand meine Finger greift" (the small hand grabs my fingers). For a band that usually sounds like a tavern brawl, it's a surprisingly soft lullaby.