Most US tap water is safe to drink, but if you have your own well or you’re unsure of where your water is coming from, there may be unknown substances in your water.
In other cases, you may just have hard water that you want to soften.
Using a water filter system such as a reverse osmosis or carbon filter will purify your drinking water.
Hard water can very easily be softened using water softeners.
Learn more about all these things here.