You must remember that a flow using NAT will look like two different flows: a flow pre-NAT, and a flow post-NAT. This is because NAT is changing one or more of the addresses in the packets. This can present a distorted view of your flows.
As Cisco explains it, NAT stitching will stitch the (apparently) separate flows to give you the single flow view:
Exporting NetFlow from the NAT devices will stitch both pre & post NAT
The Cisco Press book NetFlow for Cybersecurity goes into more detail:
Lancope’s StealthWatch solution supports a feature called network
address translation (NAT) stitching. NAT stitching uses data from
network devices to combine NAT information from inside a firewall (or
a NAT device) with information from outside the firewall (or a NAT
device) to identify which IP addresses and users are part of a
specific flow. A great feature of the StealthWatch solution is its
ability to perform “NetFlow deduplication.” This feature allows you to
deploy several NetFlow collectors within your organization without
worrying about double or triple counting the traffic.