The RNAT project was born from the wish of allowing full communication on Internet to hosts in a private network. Despite the fact that many people think it is possible nowadays, it is not true.
The priciple of communication in TCP/IP networking is based on client/server model, in short terms, to access (be a client) and to be accessed (be a server). It is not hard to see that to make it possible, each host must have an universal identification, like a house's address. For example, you can receive postcards at your home because it has an universal identification.
On the Internet, this identification is given by a number called IP (Internet protocol) address. In the beginning of the Internet, each computer had a single and valid IP, but as the Internet grew up along the years of its existence, the available amount of valid IP's started to decrease.
In parallel to this fact, small private networks were created using a private range of IP's that could not be used to access the Internet. The explanation to that is simple. Many private networks around the world could use the same IP addresses, what makes the universal identification break.
The need to make this private hosts access the Internet started to grow, forcing the development of a technique called NAT to solve part of the problem.
The NAT's idea is pretty simple. The private network hosts use a host with public and valid IP on the Internet as gateway of their communication. The gateway masks the access with its own IP address. This process, until now, helps to make the decrease of valid IP's problem smaller.
NAT seems to be the final solution to the problem, but it's not true. It just solves half problem, because it allows private hosts to act as clients on the Internet only, not servers.
This problem, up to now, has no final solution, just some patches, many times not working as expected, making us believe that NAT cannot increase the total amount of valid IP's on the Internet, once the private hosts cannot be accessed from the Internet.
RNAT is a plus to NAT. It allows hosts inside private networks to be accessed from the Internet, not breaking NAT's original functionality. This way, with RNAT is possible to have a full solution to the total amount of IP addresses on the Internet.
Some people ask me what the difference between using or not using RNAT is. There are many answers, but just to sumarize them in a simple way, it could solve the following problems: MSN's file transfer problem, FTP's timeout during the file tranfer, low-id on emule server, Connection problem on VoIP Video meeting software and more.