A Green Card is an international certificate of insurance issued by your insurance provider. It provides motorists with proof that they have the necessary minimum motor insurance cover required by the law of the country they're travelling to.
As of 2 August 2021, you no longer need a Green Card to travel to any of the following countries: Andorra, Austria, Belgium, Bosnia & Herzegovina, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Montenegro, Netherlands, Norway, Poland, Portugal, Republic of Ireland, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, and Switzerland.
Please be aware that a Green Card does not change your cover, it's evidence of the most basic legal minimum cover.