On the Tunnels page, you can configure custom svn+ssh tunnels. Tunnels are useful when already having a working SSH infrastructure which also handles authentication and communication. The configured tunnels can then be used within a Repository Profile.
A tunnel has a Name, a tunnel Command and Parameters for this command. The Command typically is an ssh executable, like PuTTY's plink.exe on Microsoft Windows or ssh on Unix and Apple Mac OS. The tunnel command is always invoked when an svn+ssh connection is set up and handles the complete SSH-communication between SmartSVN and the server. The Parameters can contain predefined variables which are expanded by concrete values from the corresponding Repository Profile on the tunnel invocation: