Hard water is water that has positively charged calcium, magnesium and iron ions dissolved in it.
Any solvent such as water can dissolve only so much of any substance. For instance, if we add salt to water, it will dissolve very easily at first. However, the more salt we add, the harder it will be for it to dissolve. Eventually, the water will reach a point where it can't dissolve more salt. Soap has negatively and positively charged ions.