A VPN = virtual private network lets you connect to a different network, usually in a different location or even country, and lets you browse as if you physically were in that network. It can be used to bypass local restrictions (e.g. website blockers) or simply to seemlessly connect to your home/office network.

There are different implementations of it, openvpn is the free/open one.

12vpn is probably one of many service providers that let you connect to their network and browse the net via their connection.