GuiXT is a client-server system based. The tool is developed by Synactive and it provides user interface customization to SAP.
To activate and get use the functionality, do the following steps,
On your SAP GUI screen, click on the icon (as per screenshot) and click again on Activate GuiXT. You will get the screen as above on the next log in.
So to deactivate, do the same step as to activate it.