Is the concentration of an aqueous solution of sodium chloride in contact with undissolved solute DIFFERENT from the concentration of a saturated sodium chloride solution in water?
In a saturated solution of a salt in water, there is equilibrium between the solid salt (at the bottom of the flask), and the dissolved salt, present in the homogeneous solution.
We could decant off the supernatant solution, and its concentration would be PRECISELY the same as it was when it was in contact with the undissolved solute.
Here we deal with a saturated solution, i.e. a solution that contains the same amount of solute as would be in equilibrium with undissolved solute. Capisce?