what is the Anhydrous

As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another. That is to say, anhydrous matter is prepared or used as a form with no water, and specifically the form of a chemical without water of crystallization.
In many cases, the presence of water can prevent a reaction from happening, or form undesirable products. To prevent this, anhydrous solvents must be used when performing certain reactions. Solvents are commonly rendered anhydrous by boiling them in the presence of a hygroscopic substance; metallic sodium is one of the most common metals used. Other methods include the addition of molecular sieves or alkali bases such as potassium hydroxide or barium oxide.