Candado translates to "padlock". It's a tangible object that immediately brings a clear image to mind, making it memorable and intriguing when found in a love song.
In the lyrics, Shakira sings, "Eso lo tenía cerrado con llaves y con candado" (I had that closed with keys and with a padlock). She uses this word metaphorically to describe how she had protected her heart and emotions after a past heartbreak, making the arrival of a new love even more impactful and surprising.
In "Nassau," Shakira sings about the thrilling but scary moment of letting someone new in after a bad breakup. She's feeling a powerful connection with someone, but she's hesitant. She tells them, "Yo que había prometido que nunca más volvería a querer" [I had promised that I would never love again]. Her heart was completely locked away, as she puts it, "cerrado con llaves y con candado" [closed with keys and a padlock]. But this new person has shown up and started to heal old wounds, and she admits, "¿qué hago? Me gustas demasiado" [what can I do? I like you too much].
Even though she's scared of getting hurt again—"tengo miedo a otra decepción" [I'm afraid of another disappointment]—the feeling is too strong to ignore. She feels like it's too late to turn back now. She asks her new love interest for a clear plan, wanting to escape with them to a remote island where they can be alone: "Llévame a un sitio sin señal y me pierdo contigo" [Take me to a place with no signal and I'll get lost with you]. It's a song about taking a risk on love again, even when you're terrified.