"Really On" is Rels B squaring up to anyone who wants to judge him. The hook is blunt: "Dime, ¿quién coño eres pa' juzgarme?" (Tell me, who the hell are you to judge me?). Coño is a strong Spanish swear word used here as "the hell," and pa' is just para (for/to) with the end clipped off. He says he doesn't think he's a star, "No me creo estrella, ni me creo importante" (I don't think I'm a star, or that I'm important), and tells you to ask his family or "la gente del parque" (the people from the park) who he really is.
The best part is his shout-out to kids from the neighbourhood: "Gente del barrio que no tiene un duro" (neighbourhood people without a penny). A duro was an old five-peseta coin, and "not having a duro" still means being broke in Spain. Some of them "dejaron los estudios pa' buscarse un curro" (quit school to find a job, curro being slang for work) to pay a mortgage their families couldn't. He also has a nice comeback for big talkers: "Si eres ladrador, cabrón, más vale que muerdas" (if you're the barking kind, you bastard, you'd better bite). It flips the saying that a barking dog never bites. The English chorus, "I've been really on", and the plea "No llores más" (don't cry anymore) give the track a softer side.