Revenue uses Digital Certificates to ensure that data is securely transmissed to their servers and that only permitted users can submit information. A digital certificate is a file containing your unique key for logging into ROS. If you try to open it in a text editor, you will only see encrypted text.
Payback uses these files to communicate with ROS. Once this is set up, you will not have to worry about it again until your certificate expires in a couple of years time.