An umbrella is sort of like a hat that you hold over your body when it's raining. It has a handle and a canopy, which is a covering made of special material that keeps the rain from getting on your head and clothes.
The canopy is usually made of waterproof material like nylon or polyester, which means it won't let any water through. And sometimes, it has a special coating on top that makes the raindrops slide off of it easily.
When you want to use an umbrella, you hold it by the handle and open it up. Some umbrellas have a button that you can press to open them automatically. Once it's open, you hold it above your head or over whatever you want to keep dry.
Umbrellas come in all sorts of colors and designs. Some are small, and some are big enough to shelter two or more people. And when you're done using your umbrella, you can close it and store it until it's needed again.