Tomato VPN turns your router into a VPN server. A VPN is a secure tunnel that allows you to connect to other networks, such as your home network, office network, database server, etc. A VPN operates on the clinet/server model. In this case, your Tomato router is the VPN server and the client is whatever computer you use to initiate the connection.
OpenVPN must be installed on both the server and the client. Tomato VPN comes prepackaged with OpenVPN. But you still need to download and install the OpenVPN software for the client computer that you will use to connect. The client computer will use either a static key or SSL/TLS for key exchange. This is necessary to validate both peers.
Tomato VPN has a built-in VPN server that is capable of taking full advantage of the most secure VPN protocol available, OpenVPN. OpenVPN uses two protocols (static and SSL/TLS). If this is your first shot at setting up a VPN server, I recommend starting out with this method. I think you will find that the “static key” is less difficult to manage.