The Host Diagnostics (HostDiag) screen shows various system information. It has been present on LG TVs since long before webOS. While it generally isn't very useful, it could come in handy in certain situations.
In general, to open HostDiag you must highlight "Channel Tuning" in the menu and press the "1" button 5 times. The menu can be opened with the menu button on the remote, but the steps after that vary a bit across webOS versions. Note that