Action / GUI Mode, Basics
Action Mode is the mode which you always start in when you spawn into the game world. It allows you to move around and perform actions in the world of Agon. The GUI Mode allows you to navigate through the Menu Bar and do many other tasks. The default key binding to toggle between Action and GUI mode is the Escape key.
Navigate to Options > Input Bindings > Gui > Gui GuiMode Toggle to change the binding or use the /bind command: /bind O Gui_GuiMode_Toggle