Answer:
Some salts have a property called "hygroscopy". This means that they absorb water from the air in a preset ratio (like, 5 molecules of water for every 2 units of the salt for example). They give this water off when heated. Therefore, there are actually two forms of the salt, they "hydrate" or "hydrous" form, which has the water, and the "anhydrate" or "anhydrous form", which does not. These two forms can have surprisingly different properties. Hydration is the process of going from an anhydrate to a hydrate (the source of the water doesn't matter, so don't say it's "absorbing water from the air").