What do you mean by hardness of water?

Hard water is simply water which has calcium and magnesium salts dissolved in it which makes it difficult to form lather with soap. While washing with hard water and soap these salts precipitate as scum and stick to the cloth. Using detergents makes it easier to form lather. Soft water is water free

The simple definition of water hardness is the amount of dissolved calcium and magnesium in the water. Hard water is high in dissolved minerals, both calcium and magnesium.