Re: A serious question.
The simplest example is rather than having a button which is 3d shaded to make it clear and distinctive that it is something special you may click on, a 'flat' design may just have a box surrounding the text which may (if you are lucky) change colour if you hover the mouse over it to show you can interact with it (particularly good when you are using touch screen style of interface). Of course other objects may also have boxes round them, or they may decide the box can be removed just leaving the text that changes colour when you realise you might be able to click over it. Similarly a flat webpage may choose to style a hyperlink so that it isn't underlined, isn't in another colour, if you are lucky it may be bold/italic but possibly only if you hover over it. Or a collapsed tree may not show any sign that it is collapsed, other than maybe the text being bold until you click on it and realise you can expand it.
Basically flat is removing any visual indicators to make it look 'clean'