Breh I didn't know what it was until about a month ago. Hard water is water that contains minerals like magnesium and limestone that doesn't get filtered out by your municipal water plant. What it does is it leaves white stains on all your pots, glasses, and stains your sink and toilet. Most of my pots are ruined because of it. Constant exposure to hard water is also harmful to your skin. That's how I found out about it. I noticed that everytime after I showered, my body was itchy all over. It also dried out my skin and discolored other parts of my body. So I have to hit up a dermatologist in the near future.
The solution to hard water is to buy a filter called a water softener but it costs too much. Plus I rent so when I brought it up to my landlord, he basically told me it's my problem. My lease is up in a month so I'm just going to find a new place to stay.