Ok sweetie, so you know how we breathe in oxygen and breathe out carbon dioxide, right? Well, sometimes when carbon dioxide mixes with water it creates something called carbonic acid.
Now, imagine if you have a glass of water and you add little drops of lemon juice or vinegar. The water becomes a little bit sour, right? That's a bit like what happens when carbon dioxide mixes with water to create carbonic acid.
Carbonic acid is a very weak acid, so weak that it won't burn or hurt us like other stronger acids can. But it's still important because it can affect the pH level of water, making it more acidic.
Carbonic acid is actually pretty important for our planet, because it's one of the ways that carbon dioxide is removed from the air and goes into the ocean. But too much carbonic acid in water can cause harm to marine life. So we need to be careful about how much carbon is released into the air and the water.