Welcome to the forum @rbrtjones188!
I’m not an authority on the matter nor am I in any way associated with PSA. Thus I can only offer my understanding of the ordering process based on what I’ve experienced myself and read here in the forum. If you’re okay with that, read on…
When your order is “In Review” it means PSA’s compliance department is validating the purchase to ensure the item or items are legally allowed to be owned and shipped to your state / local area of residence. There may be other things involved in this process like vetting your FFL and their paperwork, and so it seems legal compliance is the consensus as to primarily what’s going on behind the scenes when an order is “In Review” . It takes some time for the review process to complete and can vary greatly based on the volume of orders PSA has in the pipe.
Here’s a recent thread on the subject. It comes up here on the forum quite often.
Others please correct me if anything I’ve stated is incorrect.