The Debug -> Web farm tab is displayed only when the system is running in web farm environment and when web farm debugging is enabled. This can be achieved either by configuring certain settings in Site Manager -> Settings -> System -> Debug, or by adding specific keys to the appSettings section of your project's web.config file. Both options are described in this context help topic.
On the Debug -> Web farm tab, you can see which web farm tasks were logged and which servers were notified about the changes. This is particularly useful if you want to find out if the web farm works effectively and synchronizes the correct data, or generally if you want to troubleshoot web-farm-related issues.
The bottom part shows the logged tasks, while the top part shows asynchronous notifications of other web farm servers. Tasks with the DO: prefix are synchronization tasks received from other servers which were (are being) processed.
Enabling the Show complete context check-box displays complete context (not only the topmost item) in the Context column. The log can be cleared using the Clear web farm log button.
For more information about debugging in Kentico CMS, please refer to Developer's Guide -> Development -> Debugging and system information. Detailed information about the Web farm synchronization module can be found in Developer's Guide -> Modules -> Web farm synchronization.