A virtual server is a group of physical servers from which the best rated server will automatically get preferred at Job execution time. In Talend Administration Center, simply select physical servers and group them into a virtual server. Then set the execution task onto this virtual server as for any execution server, in order to automatically select the best server to execute the task onto.
Access to the Virtual servers page depend on your license. For more information, see What modules and features are available depending on your license.