Un cero a la izquierda literally translates to "a zero on the left". It's a fantastic Spanish idiom used to describe feeling worthless or insignificant.
Just like how a zero placed to the left of a number (e.g., 07) has no value, the singer feels completely unimportant without his love. He sings "soy un cero a la izquierda" (I'm a nobody) to express the depth of his sadness and how the breakup has shattered his sense of self-worth.
In "Faltas Tú", Colombian band Morat brings their signature upbeat folk-pop energy to a brutally honest post-breakup spiral. The narrator feels completely stripped of purpose, tossing out absurd comparisons like being "como galaxia sin estrellas" ("like a galaxy without stars") or "como un zapato sin su media" ("like a shoe without its sock"). Parties feel totally pointless when every phone call goes straight to voicemail. He admits he deleted only one text message: "ese con el que te despediste" ("the one where you said goodbye"), simply because staring at it hurts too much while he runs on just "diez por ciento de energía" ("ten percent energy").
The track is packed with colorful everyday Spanish idioms that show how low his mood has dropped. He calls himself a "cero a la izquierda" (literally "a zero to the left", a common idiom meaning a complete nobody or totally useless), and bluntly complains that "salir me sabe a mierda" ("going out tastes like shit" / "going out sucks completely"). Despite feeling like a ghost wandering into empty echoes, he refuses to move on, stubbornly insisting he will not be "el perdedor que no te insiste" ("the loser who doesn't keep insisting"). It is a funny, desperate, and stubbornly loyal snapshot of someone refusing to accept that it is over.