Okay, kiddo! So, an inch of water is like a way of measuring how much water there is. Imagine you have a big long straw and you put it in a cup of water. Now, if you suck on the straw really hard, some of the water will go up the straw and you can drink it! But if you stop sucking, the water will go back down the straw and into the cup.
Now, let's say we have a big swimming pool outside. If we took a ruler and measured how deep the water is, we could say it's 3 feet deep or 5 feet deep or whatever. But sometimes we want to be more specific about how much water there is. That's where an inch of water comes in.
It's like taking a really thin sheet of water and measuring how much there is. If we poured a sheet of water over the ground that was one inch thick, and then measured how big the puddle was, we'd know exactly how much water fell there. It's like putting a tiny ruler in the water and seeing how high it goes!
So really, an inch of water is just a way of measuring how much water there is in a specific area. But instead of measuring the depth of the water, we're measuring how much it covers everything underneath it!