Huevos literally means "eggs," but in Mexican Spanish slang, it's a common and colorful way to say "guts" or "courage." It implies a strong sense of determination and bravery.
In the context of the song, the artist sings, "Mas no ven que le metí huevos y mente" (But they don't see that I put in guts and brains). This phrase powerfully conveys that his success isn't just luck, but the result of hard work, courage, and intelligence, making it a very engaging and culturally rich word choice.
The whole song is one question turned back on the people who asked it: "¿Cómo fue? Que le está yendo bien al loco" ("How come? That the crazy guy's doing well"), about "Aquel que ayer pintaba para ser muy poco" ("The one who yesterday looked like he'd amount to very little"). The answer is money, named in three different slangs inside a single verse — el papel, el queso, la feria — and the pleasure of having it is mostly the pleasure of silence: "El hocico les tapé a todos" ("I shut everybody up"). By the end of the verse the brag has run past spending altogether: "Tanto que ya no tengo ni en qué gastarlo" ("So much that I don't even have anything to spend it on").
Then the chorus pulls the other way. "Pero, aunque ahorita hay con queso / No crean que por eso la feria podrá cambiarme" ("But even though right now there's cheese / Don't think that because of that the cash can change me"), because "Sé lo que es no traer un peso" ("I know what it's like to not have a peso") and "del ayer no he de olvidarme" ("I won't forget yesterday"). The second verse answers the one accusation that stings, which is that it was luck: "Todos creen que del cielo caen los billetes" ("Everybody thinks that bills fall from the sky"). His reply is a list of what actually went in — "huevos y mente" ("balls and brains") — plus the family that made him strong.