Fierro is a fascinating word because of its multiple meanings in Mexican slang. While its literal translation is "iron", it is most commonly used in this context to mean "gun".
This song cleverly uses both the noun and exclamation forms. The lyric "Moon Rocks junto al fierro" (Moon Rocks next to the gun) points to a dangerous lifestyle. However, when the artist shouts "¡Fierro!", it's an energetic exclamation meaning "Let's go!" or "Right on!", making it a key piece of slang for understanding this genre of music.
Tito Double P lays out the highs and lows of his new life in "MR INTERNACIONAL". He's living fast, driving at "Ciento ochenta" (one hundred eighty) without even looking at the wheel, but he also admits, "Extraño mi vida de antes" (I miss my old life). Fame has brought a lot of changes, and not all of them are good. He paints a wild picture of his current reality: a bible on the dashboard right next to "Moon Rocks junto al fierro" (Moon Rocks next to the gun). It's a life of luxury, with exclusive Moncler outfits and friends from Spain bathing in champagne in Cabo, but it's also a life where everyone has an opinion on his money, "Como si fuera de ellos" (As if it were theirs).
The song is basically a tour of his new status as "Mr. Internacional viajando el mundo entero" (Mr. International traveling the whole world). He's constantly flying, eating lobster on the coast, and riding jet skis. He knows people doubt him—he sees the "millón de comentarios diciendo 'no creo'" (a million comments saying 'I don't believe it')—but he just dismisses them. Now, wherever he goes, he hears "puro 'yes, sir'" (only 'yes, sir') and gets to do whatever he wants. It's a blunt look at the strange world of being rich and famous.