Doncella translates to "maiden" or "damsel". It's a beautiful, old-fashioned word that you might find in fairy tales about knights and princesses.
In this bachata ballad, Romeo Santos uses mi doncella (my maiden) as a term of endearment. It's a deeply romantic and chivalrous way to show that he sees his love interest as pure, precious, and worthy of honor, elevating her above all others.
This song is a duet about a love so huge it feels like it could span the universe. Romeo Santos and Monchy & Alexandra trade lines, each describing the other as their everything. It's full of the kind of grand, romantic statements that Romeo Santos is famous for. He doesn't just love her, he wants to "agregarte a la biblia por divina y tan buena" (add you to the bible for being so divine and good). It's a huge compliment!
They see each other as perfect, even in their flaws, calling her "la mujer de mis sueños con imperfecciones perfectas" (the woman of my dreams with perfect imperfections). The love they share is described as a guiding force, a source of peace, and something that will last forever. They declare their love is measured in "años luz" (light years), a poetic way of saying it's immeasurably vast. It's a beautiful back-and-forth between two people who believe they've found their one true person.