IP address and netmask related calulations.
The box shows your current IP adderss observed by this tool and it is your WAN/Internet-facing IP address. It might be different from your network interface's if you are behind a SOHO Router or firewall (or techincally NAT).
IPv6 is a version of the Internet Protocol suceceeding IPv4. The Internet is still mostly built on IPv4 so it is ok if you do not have IPv6 connectivity. IPv6 allows every device to have an unique IP, eliminating the need of NAT and its shortcoming, by providing a much larger address space than IPv4.
If both IPv4 and IPv6 boxes are showing an valid IP - your host is dual-stacked and could access both type of host on the Internet. In this case, the primary IP address box shows which IP network your host prefers and is almost always IPv6.
If you are connecting through a proxy server, the IP shown might actually be the IP of the proxy. Yours IP might be listed in the X-Forward-For box.
Address within valid address range is a valid address assignable to network interface.
Traffic addressed to the broadcast address would be routed to every device in this network.
The result is the smallest subnet which has both address 1 and address 2 in the same subnet.