Like another famous children's song, "Au clair de la lune", it has an adult theme - in this case, one of lost love. The song speaks of a lover bathing in a fountain, hearing a nightingale singing, and thinking about her long lost lover whom she lost after failing to give her rosebud. The nightingale's heart laughs but hers weeps. The rosebud is a euphemism for maidenhood, and thus she wishes it were still intact and could still be given to her long lost lover.
The refrain is repeated at the end of each verse:
“Il y’a longtemps que je t’aime, Jamais je ne t’oublierai.”
“Long have I loved you, Never will I forget you.”