Doncella translates to "maiden" or "damsel". It's a beautiful, somewhat archaic word that evokes images of classic romance and chivalry, making it stand out from more common terms of endearment.
In the song, Ricardo Montaner uses doncella to describe his beloved as a pure and cherished figure who cares for him, singing "Doncella que me cuida y me despeina" (Maiden who takes care of me and messes up my hair). This choice adds a poetic and timeless quality to his expression of love.