What is Propositional Representation?

Jun 2021
So I read that there are basically two modes of thinking, one is visual and the other is in words which is called propositional thinking.

How is concept related to them?
Furthermore, there is another kind of propositional theory mentioned here in which the researchers said that it is neither images nor words but a smallest proposition.
Can anyone explain this part to me.
I will also add the book page from which I am studying

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