Cobre literally means "copper," a common metal. However, its power comes from the classic Spanish idiom used in the song: "cambiar el oro por el cobre" (to trade gold for copper).
Daddy Yankee uses this phrase to express his deep regret after a breakup. He realizes he made a terrible mistake, trading something invaluable and real (his partner, the "gold") for something of much lesser value (meaningless flings, the "copper"). It's a beautiful, memorable metaphor for making a bad trade in love.
In "Vuelve", Puerto Rican reggaeton heavyweights Daddy Yankee and Bad Bunny team up to beg an ex to come back after realizing they ruined a great relationship. The title literally means "Come back," and both artists admit they messed up big time. Daddy Yankee lays down the central metaphor at the end: "Cambiamos el oro por el cobre" (We traded gold for copper), meaning he foolishly threw away something priceless for someone of lesser value. Now that she has moved on to someone new ("Estás con otro, ya yo me enteré" / You are with someone else, I already found out), he is stuck dealing with regret, wondering if this new guy can satisfy her the way he used to.
Bad Bunny jumps in with a mix of celebrity shout-outs and sharp Puerto Rican slang to show how irreplaceable she was. He compares their past chemistry to famous power duos: "Tú eras mi Angelina, yo era tu Brad Pitt" (You were my Angelina, I was your Brad Pitt) and "como LeBron y D-Wade en los Heat" (like LeBron and D-Wade on the Miami Heat). He complains that he found out she has a new guy ("otro gato", Puerto Rican slang for 'another boyfriend' or 'another guy'), while dismissing casual flings as "Louis Vuitton feka" (fake Louis Vuitton bags made in China). The whole track is a frantic plea to fix a bad gamble before time runs out.