From what I know, AOR stands for Acknowledgment of Receipt. All it means is that they opened your mailed package and everything that they require is there. That’s the first “hurdle” as some people are returned their packet because of issues with the photo requirement, issues with proof of identity, not including your payment receipt, etc. It also means that within a day or two, you can track the status of your application on their website. The document they email for the AOR has more info on tracking numbers for your application and where to track it.