"Carta Verde" (or "Green Slip", in a free translation) is a mandatory insurance coverage available to customers traveling by car from Brazil to other Mercosul countries (Argentina, Uruguay, Paraguay and Venezuela).
Not all rental companies are allowed to issue this insurance. You must check with the company of your choice if they can issue this document if your travel plans include crossing the borders of the afore mentioned countries.
You must also check if the coverage and protections included in this insurance are valid also in the countries you will be driving to.