Desarmar literally means "to disarm" or "to take apart". It's a powerful and versatile verb that's often used poetically in Spanish.
In the song, Reik sings "El corazón se me desarma", which translates to "My heart falls apart" or "My heart is disarmed". He uses this image to express a beautiful sense of vulnerability, as if love is dismantling all his defenses.
This is a massive love song about being completely saved by someone. The singer starts by saying that all the lonely nights and wrong turns he took in the past don't matter anymore. Why? Because just when he was about to give up, he found this person. He literally says, "en un segundo de rendirme te encontré" (in a second of giving up, I found you).
This new love has totally transformed his life. It made him "indestructible" and "detuvo mi caída libre" (stopped my free fall). He feels so safe that he calls this person his "único lugar seguro" (only safe place). All the pain and personal demons from before are gone; he says his pain is "kilómetros atrás" (kilometers behind) and his ghosts, "hoy por fin están en paz" (are finally at peace today).