Prolific Completion Code: What is it and where do I enter it?

At the end of the study it says close the window, so I do that. Then it says my completion of the study is "successfully logged" and "no further action is necessary" but then it gives me a completion code. What is this for? I don't want to mess up something and accidentally not earn the money. Do I have to do anything with it?

Thanks for your help. If you can't tell I am quite new.