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 is not known for being subtle, and this song is a perfect example! He's singing to an ex who has moved on with a new boyfriend, but he is completely convinced she still belongs to him. He's heard her new guy is an “insípido aburrido” (a bland, boring guy) and that she's a “fogata” (a bonfire) while her new man is “tan frío” (so cold). He basically thinks this new relationship is a joke.
The whole song is a warning to her and her new partner. Romeo declares, “Eres mía, mía, mía” (You're mine, mine, mine) and promises, “No te asombres si una noche / Entro a tu cuarto y nuevamente te hago mía” (Don't be surprised if one night / I enter your room and make you mine again). He even has a plan for her wedding day: he'll laugh and tell her new husband that she's only “prestada” (borrowed) because she will always be his. It's incredibly arrogant, but that's why people love the self-proclaimed King of Bachata!