Arcobaleno literally translates to "rainbow". It's a beautiful and evocative word that paints a vivid image.
In the song, it's used metaphorically in the line "Questo grande arcobaleno non può sciogliere un veleno" (This great rainbow cannot dissolve a poison). This suggests that even something as beautiful and hopeful as a rainbow cannot overcome the deep-seated pain or 'poison' in the relationship, making it a poignant and memorable choice.
This duet is a conversation between two people whose relationship is ending, and it's incredibly dramatic! They know it's over, but they just can't seem to let go. They describe their love as a poison they have to drink together, saying even a "grande arcobaleno" (big rainbow) can't fix things. It's a powerful way to say that even the beautiful memories can't heal the damage.
The whole song is this back-and-forth of pain and desperation. They agree that they can't erase their "grande amore" (great love), but they're stuck. The chorus is the heart of the problem: "Un cuore malato fa male perché / Batte forte soltanto per te" (A sick heart hurts because / It beats strongly only for you). They feel like they are physically ill with love for each other, even when they know "una speranza non c'è" (there is no hope). It gets even more intense when one says "Io posso morire" (I can die) and the other begs them not to say it. It's a perfect song for when you want to feel all the feelings about a love that's impossible to leave behind.