Port forwarding allows you to let certain services through the firewall to your computer. This is useful for remote access, hosting game or other servers, or for things that require you to accept incoming connections.
Using it is very simple. Log into your VPNme account and find the settings for your VPN tunnel. Make sure you're using direct mode and type the port number you want forwarded in the box. From the list that pops up, select the port and you're done. Any changes will be applied the next time you connect.
If you're unsure of whether you need the UDP or TCP port, it's safe to allow both. To remove a forwarded port, just click the x next to it.