IPv6 is version 6 of the Internet Protocol. It's the successor to IPv4, which is the current networking protocol in use on the general Internet. (In case you're wondering, IPv5 was never deployed; it doesn't exist on the Internet.)
Motivation for IPv6
The primary IPv6 feature driving its deployment is vastly larger address space, solving the IPv4 problem of not having enough addresses for everything which people want to connect to the Internet. There are other technical features of IPv6, some of which are fixes for deficiencies in IPv4.
See the articles below for more information about IPv6