Suppose young Timmy mistakenly takes 'prime number' to be roughly synonymous with 'cool number'. So he goes around saying things like '666 is a prime number'. Does he believe that 666 is a prime number? Presumably not. He certainly doesn't have a de dicto belief involving the concept prime number, since he lacks this concept (he associates the words, 'prime number', with a different concept entirely). Nor does he have any de re beliefs about primes, i.e. beliefs which talk about this property under a different guise: he does not believe, for example, that 666 is divisible only by itself and 1. What Timmy believes is that 666 is a cool number (or, more likely yet, that '666' is a cool numeral), and he mistakenly takes the sentence '666 is a prime number' to express this belief. (source)
As we can see an utterance (an expression) has a meaning behind it. Usually, meanings are the goal and expressions are merely means to achieve that goal. Thats why, it is important that when we hear an expression, we analyze it and start digging for the meaning behind that expression.
The problem is, as humans we are incapable of communicating meanings, we are only capable of communicating expressions. This realization leads to The Problem of Representation (aka representational skepticism) that I have discussed before.
Lingual skepticism and other forms of skepticism like brains in a vat skepticism, and The Matrix skepticism are closely related. They all go around the limitations of empiricism, and the limitations of our senses to grasp reality.
In this series:
Expressions And Meanings - Part 1: Introduction
Expressions And Meanings - Part 2: Elaboration
Expressions And Meanings - Part 3: Linguistic Relativism
Expressions And Meanings - Part 4: Conclusion