When a hydrophilic substance like Na+Cl- dissolves in water, electrostatic interactions occur between the partial negative and positive regions of the water molecules and the hydrophilic substances.
The result of this charged interaction is two classes of water molecules:
1) those that are electrostatically bound to the dissolved substances and
2) those that form hydrogen bonds only between other water molecules.
The water molecules that form eletrostatic interactions between the dissolved substances are called bound water.
The water molecules that hydrogen bond to only other water molecules are called free water.