The Meridian VPN runs on OpenVPN.
Note that if you install OpenVPN on Windows, it must be run as an administrator to work correctly.
If you have an account with Meridian, you can have an OpenVPN profile emailed to you by going to the VPN request page and logging in. If you don't have an account then you won't be able to connect.
Using the VPN allows you to host 'LAN' games which are available to everyone else on the VPN. This makes it particularly easy to host servers. The VPN also allows special access to some internal services available from Meridian.
Note that the VPN Meridian uses is not the kind that hides your internet traffic.