Celosa means "jealous" in Spanish. While jealousy is a common human emotion, its use in this song is quite unique and personified.
The lyrics state, "Tanto que la luna se puso celosa de esa princesa" (So much that the moon became jealous of that princess). This poetic imagery attributes human emotion to the moon, making the word stand out and creating a vivid, memorable picture of the subject's overwhelming beauty.
This song is a passionate declaration of love at first sight. The singer confesses that even though they haven't known each other for long, the person he sings to has already captured his heart and all his happiness. He can't stand waiting any longer to finally meet them, as he's already imagining a beautiful future together and is ready to give his entire life to them.
The central theme is the breathtaking beauty of "Lucía." The singer uses a powerful and romantic image to describe it:
*"Lucía tan bella *tanto que la luna se puso celosa"
This translates to "You looked so beautiful that the moon got jealous." Her beauty is so radiant that it outshines the moon, causing it to hide for the entire night. It's a poetic way of saying she is the most beautiful thing he has ever seen, making it impossible for him not to gaze into her eyes and dream of a life with her.