This refers to the process where anhydrous (water-free) copper sulfate dissolves in water, forming hydrated copper sulfate.
Hydrate: A compound that includes water molecules within its crystal structure; hydrated copper sulfate is an example.
The maximum amount of a substance that can dissolve in a certain amount of solvent at a specific temperature.
The substance being dissolved in a solution; here, it would be the anhydrous copper sulfate.