In computing a "tunnel" is where a remote service or computer, that would otherwise not be accessible, is made available to a local network.
The example that most people are familiar with is the Virtual Private Network or VPN. This allows you to "tunnel" from you workplace network into another remote network via a secure tunnel. However a VPN is not always required and in fact may provide too much access to the remote network creating a security risk.
Let's say there is a web server running at a remote plant that I wish to access. I could use a VPN, but this is overkill and creates an unnecessary link between my local network and the remote one. Instead I want to access the web server as if it was running on my local network. This is where the Ardexa TUNNEL feature can help.