Chavalón is a colloquial term, particularly common in Mexico, that refers to a young man or a 'dude'. It's a more informal and endearing way to say 'joven' or 'muchacho'.
In the song, it's used to describe "el compa Markitos" as "siempre al tiro el chavalón" (always on point, the young man), highlighting his readiness and perhaps his youthful energy within the context of the corrido. It adds a touch of regional flavor and familiarity to the lyrics.
The song announces its own purpose partway through: "Ahí le va su corrido compa Markitos" ("Here goes his corrido, buddy Markitos"). Everything before that line is the portrait — a friend seen around Culiacán, "En su gladiador lo han de mirar" ("In his gladiator, they must see him"), police-scanner chatter left on for entertainment, "Policiacas pa' cura sacar", and a young man described as "Siempre al tiro el chavalón" ("Always ready, the young man"). The standard Gabito holds him to is set in the first two lines: "Estamos pa' sumar / Jamás pa' restar" ("We are here to add / Never to subtract").
Halfway through, the voice stops describing and starts answering: "Mucha gente me critica / Sin saber de mi pasado" ("A lot of people criticize me / Without knowing my past"), then "No le debo nada a nadie / Por qué todo me he ganado" ("I owe nothing to anyone / Because everything I have earned"). This is the En Vivo take, recorded with the crowd audible behind him, and it closes on the line meant to leave the room with the audience: "Jamás la mirada hay que bajar" ("Never should the gaze be lowered").