In sections 139-142 are some of the most interesting so far. Here he uses the example of the cube to investigate the relations between the picture we get of a word and how we may use it. That is, how does the picture we get of a cube really limit the way we may use it.
One may think that the picture we have of a cube will completely determine how we use the word. But, we will be surprised when we can think of a triangular projection of a cube and still have to call it a 'cube'.
It is too easy to just say, as everyone does these days, that the USE of a word is what we mean by it. Wittgenstein, pun intended, turns it around in his mind and shows how the notion of 'use' reveals potentials in a word beyond what we immediately think when a picture occurs in our mind. We may think we mean the picture by the word, and that has to be an important part of it, but it is not the full expression of the word. It is the place the word in all our language games that reveal its potential.
We do not immediately think of all possible uses of a word when we hear it, in that regard the picture is a central part feature of the 'meaning' of a word. Perhaps the 'meaning' of the word is not something that happens in a single instant when we hear it, and we should think of the meaning of a word as something extended in time, that mutates, expands and contracts. Now, this does not mean we mean NOTHING by the word, only that the meaning of a word is a complex of things 'before the Mind' and uses we make of it. The picture that comes to mind might always be the same, so doesn't change with time, but we use the word in time in new ways and contexts that the image in our mind alone does not immediately suggest. Wittgenstein emphasizes that the 'meaning' of a word is tied to how it is used in practice, which in turn depends on how we are trained to participate in language-games.