Criptonita is the Spanish word for "Kryptonite," the famous fictional substance that is Superman's only weakness. It's a perfect example of a modern, pop-culture word being used metaphorically in a song.
In the lyrics, the singer calls a person his criptonita to express that they are his one great vulnerability. Despite his tough exterior, this person is the only one with the power to hurt him, making it a powerful and memorable metaphor for a love that makes you weak.
This song is a raw confession about a breakup that has left the singer completely wrecked. He admits his heart hurts from another betrayal, telling his ex, "ya ni me beses" (don't even kiss me anymore). The relationship didn't even last three months, but she's his weakness, his "criptonita" (kryptonite). Even though he says she's nothing special, he also calls her his favorite.
He's dealing with the pain by getting wasted. He describes himself "vaciando unos botes y viendo a la luna" (emptying some cans and looking at the moon). He feels like he lost his mind looking into her eyes and now wants to stay in that madness. He tries to act tough, saying he's a "cabrón" (a badass) and that she'll be the one crying, but he also admits, "me lastimas" (you hurt me). It's a classic song about trying to numb heartbreak with partying.