“Carrier-Grade NAT” is basically where the “Internet provider” issues their customers private IP addresses for them to use as their “Public IP”:
Carrier gets a Public IP 64.x.x.x, NATs it to their customers as, say, 10.1.x.x. The customers “home routers” NAT it again to another Private IP range – 192.168.x.x.
Its an attempt to styme the effects of IPv4 Public IP depletion and/or ease the transition to IPv6…
See also: Carrier Grade NAT