What will be happens when "Open on mouse over" in preferences is active? How does it work can you give me an example please, Thanx
'Open on mouse over' is for menus.
This option allow to open a menu, just because the mouse cursor is over it.
If the option is not checked, you need to click on menus to open them.
Most of things are documented in Blender. If you keep the mouse cursor above the option, you'll have some explanation