Trying to figure out how to describe AOR

I've read through the faq, and I'm signin struggling to be sure what an AOR is and means. I'd assumed an AOR just means are they're saying we've received it, and looked at it enough to know they aren't sending the packet back to you. I've also seen people saying they have a receipt number but no AOR yet. Is an AOR further along in the process than I realized?