You have your notebook with you. We know it has 6 faces. Top, bottom, one thin one at left, one at right and front and back faces.
Now you throw the notebook. We have sample space S = {1,2,3,4,5,6} where these numbers represent each of the six faces of the book on which it may land (in final stable state) when you throw it.
All the outcomes in the sample space are equally likely.