Water is corrosive. When you run it through lead pipes, it dissolves some of the lead and carries it into the water supply that we drink and bathe in. That's bad. Lead is poisonous.

The good news is that municipal water supplies have been adding phosphates to water for decades. The phosphates react with the lead to produce a scale layer inside the pipe. That prevents the water from corroding the lead, making the water safe to drink.

Detroit's municipal water supply, which served Flint (and basically the entire metro-Detroit area), had been doing exactly that. Flint was fine. Until they had the idiotic idea to change their water supply from Detroit's to their own water system using the Flint river. When they changed over, the water chemistry changed (because it's a different source), and also because they didn't use corrosion control additives. So the protective coating inside the pipes dissolved, and Flint's water supply became contaminated with lead.

Flint's incompetent leaders broke a safe and working system in the name of trying to save a few bucks, and poisoned the city in the process.