Lastre literally means "ballast," the heavy material used to stabilize a ship. However, it's often used metaphorically to mean a "burden" or "hindrance."
In this song, the lyrics ask, "Yo no sé si soltar el lastre y no volver jamás" (I don't know if I should release the burden and never return). This powerful imagery suggests letting go of something heavy and difficult, making it a unique and memorable word choice that adds depth to the song's emotional struggle.
Reik and Leo Rizzi set this one on an actual beach, and the sand is not a metaphor for anything. The singer is standing on it waiting for someone who sailed off: "Apagaste el faro y no te pude encontrar" (You turned off the lighthouse and I couldn't find you). Everything around him then gives him contradictory advice. "Y ahora un aire me susurra, 'Volverá, volverá'" (And now a breeze whispers to me, 'He'll return, he'll return'), and two lines later "Y los pájaros anuncian, 'No esperar y no esperar'" (And the birds announce, 'Don't wait and don't wait'). The breeze says stay, the birds say leave, and he does neither.
The chorus is supposed to be the decision. "No sigo esperándote aquí en la arena" (I'm not waiting for you here in the sand anymore) sounds final until you count how many times he repeats it. The honest line is the quiet one just before it: "Yo no sé si soltar el lastre y no volver jamás" (I don't know whether to let go of the ballast and never come back). Lastre is the weight a ship carries to keep itself upright, so dropping it means going faster and losing your balance at the same time. That is the argument he is having with himself for the whole song, in one word.