Joan Sebastian takes a heartbreak and turns it into geology. The song starts in the past tense: "Yo tuve corazón para quererte" (I had a heart to love you), "para morirme" (to die for you), "para llorarte" (to cry over you). Then comes the turn, "No tengo corazón pa perdonarte" (I don't have the heart to forgive you), because "Se me hizo roca el corazón" (my heart turned to rock). The pa is just a spoken shortening of para.
The rock heart is not a sad ending, though. Nothing grows on it, "ni las flores ni la hiedra" (neither flowers nor ivy), no random kiss can fool it any more, and even though it is stone "Es transparente" (it's transparent). His conclusion is almost smug: "Salí ganando, el corazón es un diamante" (I came out winning, my heart is a diamond). The last line explains the title: now he knows "cuándo es amor y cuándo es trampa" (when it's love and when it's a trick). A trampa is a trap or a con, and here it means a love that was never honest.
Useful pattern: se me hizo (it became, on me) shows how Spanish uses se plus an indirect pronoun for things that happen to you.