Here you can put some settings about the count of threads of the Jetty HTTP server in the Mendix system. You use this only when connection problems occur on a server with much HTTP traffic.
For more information, see https://world.mendix.com/display/howto25/Configure+the+Mendix+application (search on Jetty).
These settings are specific to our embedded Jetty implementation. There are more settings possible for our embedded implementation, but at this moment, Mendix offers only the settings as described in the documentation to be altered. In most situations, these settings are sufficient. To reduce the load of the embedded Jetty server, it is always good to use a separate web server (IIS/Apache) to serve the static content.