Prestada is the feminine form of the word for "borrowed" or "on loan". It's a common word, but Romeo Santos uses it in a shocking and possessive way in this song.
In the lyrics, he imagines telling the woman's future husband that the woman he loves is only prestada (on loan). He's claiming that her new marriage is temporary, because in his eyes, she will always truly belong to him. This audacious statement perfectly captures the song's dramatic and obsessive theme.
Romeo Santos brings maximum confidence and dramatic possessiveness to "Eres Mía", a bachata track all about refusing to let an ex move on. He starts off sizing up her new boyfriend, calling him "un insípido aburrido" (a bland, boring guy) and comparing her fiery personality ("tú que eres fogata" / you who are a bonfire) to the new man's cold demeanor ("él tan frío" / he is so cold). Romeo admits straight up to his own flaws, singing about "el egoísmo de ser dueño de tu vida" (the selfishness of owning your life), but he does not back down. He even jokes that if she walks down the aisle, he will laugh and tell the groom that his bride is only on loan.
Language learners will love how Romeo plays with common Spanish sayings here. He tackles the famous proverb "un clavo saca otro clavo" (one nail drives out another nail, which means a new lover helps you forget an old one) and directly rejects it: "Dicen que un clavo saca un clavo, pero eso es solo rima" (They say one nail drives out another, but that is just a rhyme). He insists no tool exists that can pry his love away. Crowned in his own signature nickname "The King", Romeo leaves no room for doubt about who he thinks she truly belongs to.