Sonámbulo translates to "sleepwalker." It's an evocative and somewhat uncommon word in song lyrics, making it stand out.
In the song, the artist uses it to describe his desperate search for a lost love: "Sonámbulo te busco y no puedo dormir" (Like a sleepwalker, I look for you and I can't sleep). This metaphor beautifully conveys a state of restless longing and an almost unconscious, driven search for the person he misses, even while awake.
The whole song hangs on one small modern humiliation: "Otra vez que me dejas en visto" (again you leave me on read). No fight, no goodbye, just a message she saw and never answered. Ken-Y and Farruko build the rest of the night out of that silence. He cannot sleep, so he wanders the house instead: "Sonámbulo te busco y no puedo dormir" (sleepwalking I look for you and I cannot sleep). She only turns up in his head now, and the chorus keeps having to admit it out loud, "Que es solo un sueño" (that it is only a dream), "Que no te tengo" (that I do not have you).
The devotion gets oddly specific, which is what saves the song from being ordinary heartbreak. He does not say she had his heart, he says she had his passcode: "De mi celu, y que sea la única que llame" (of my phone, and she is the only one who calls). A verse later he offers to do prison time for her, "Por ti te hago tiempo hasta de preso" (for you I would even do time locked up). It is too much. He knows it is too much. The beat stays soft underneath it, so the line lands as devotion rather than swagger.