Madrugada refers to the very early hours of the morning, typically from midnight until dawn. It's a more specific and evocative term than just "morning."
In this song, the artist uses it to describe being awake "toda la madrugada" (all through the early morning), unable to sleep because he's consumed by thoughts of a past love. It paints a vivid picture of sleepless nights and longing, making it a memorable and relatable word in the context of heartbreak.
Dalex opens this one wide awake at 4am, popping pills that do nothing and stewing over how easily his ex seems to have moved on. "Dime, ¿cómo lo haces tú?" [Tell me, how do you do it?] he asks, half admiration and half resentment. The best line in the song is the joke he makes at his own expense: "Sin ti soy como un artista sin autotune, como Luny sin Tunes" [Without you I'm like an artist without autotune, like Luny without Tunes], comparing himself to half of the reggaeton production duo Luny Tunes, useless without his other half.
He's not over her and he knows it. He still checks her photos and hates it when he's not in them, "Odio cuando subes fotos y no son conmigo" [I hate when you post photos and they're not with me], and he keeps watching from a distance even though he's stopped chasing her, "Aunque no te siga, yo siempre te miro" [Although I don't follow you, I always watch you]. The title line, "Porque nadie sabe lo cabrón que se siente" [Because nobody knows how damn awful it feels], is the closest he gets to admitting how bad it actually is under the bravado. He'd rather dream about kissing her than sleep at all.