This is a transition mechanism in which a gateway server fills a role similar to an IPv4 NAT box, except that it is also translating protocols as well as addresses.
The mechanism is defined in RFC 2766 and although the IETF has deprecated it to historical status, it is likely to be widely used until the balance of Internet sites are accessible through IPv6.
There are alternatives to NAT-PT being developed such as IVI (roman numerals for iv and vi). This was developed in China where there is already a large IPv6-only Internet backbone covering 18 cities. For more check these slides from IETF OPSAREA. Also, Kernel patches are available for an IVI server for Linux 2.6.12 and 2.6.18.
NATPT is likely to be replaced by NAT64 now that the IETF is taking this type of transition gateway seriously.