From XnView Wiki
XnView presents two different user interfaces, or windows, depending on context: Browser Mode and Viewer Mode. When XnView is started, it will normally begin in Browser mode and show the contents of a folder.
XnView's menu bars and tool bars are context-sensitive, and change depending on the mode The toolbars are also dependent on context. Grayed-out menu items are not available in the current mode or context.
Most of XnView's common functions and menu/toolbar items are available using Shortcut Keys.
Tool tips are available throughout XnView. To display the function of a toolbar icon, simply move the mouse over the button and leave it there for a moment.
A third display option, Basic Mode, is rarely need or used, but is mentioned here for purposes of being thorough.