cero a la izquierda is a fantastic Spanish idiom that literally translates to "a zero on the left". Think about it: a zero in front of a number, like in '07', adds no value.
Figuratively, it means something or someone is worthless or completely insignificant. In the song, Shakira sings "el pasado es un cero a la izquierda" (the past is a zero on the left) to powerfully express that she's letting go of what's behind her and focusing only on the present and future. Learning idioms like this is a great way to sound more like a native speaker!
Shakira cuts straight through everyday stress with a blunt reminder that life can be rough: "La vida es una perra, ya lo sé" ("Life's a bitch, I know"). Instead of letting small annoyances ruin the day, she suggests packing a towel and swimsuit to escape to the beach, forgetting the world's problems even if it is just for a "milimicrosegundo" ("micro-millisecond"). The heart of the song is the classic truth that time flies when you are having fun ("El tiempo se va volando / Cuando se pasa bien"), and the circumstances do not matter at all. The "cómo, dónde y cuándo" ("how, where, and when") take a back seat to "con quién" ("with whom").
She also shares some sharp observations, declaring that between city lies and polluted oceans, the only honest people left are "los borrachos en los bares" ("the drunks in the bars"). She drops a great Spanish idiom when she sings that "el pasado es un cero a la izquierda" ("the past is a zero to the left"), which means something is completely worthless or irrelevant, like adding a zero before a whole number. Once you drop that dead weight ("Me quité lo que me pesaba"), yesterday is gone, tomorrow is not here yet, and all you really have is today.