Imagine you have a piece of paper that you write on with a pencil. A slate is kind of like a bigger, fancier version of that paper. It's usually made of a smooth, flat piece of stone or another hard material. People used to use slates as a way to practice writing or do schoolwork before we had paper and notebooks like we do now.
The cool thing about slates is that you can use them over and over again. Instead of using a pencil, you use a special tool called a stylus to write on the slate. When you're done, you can wipe the slate clean with a cloth or your hand and start again. It's kind of like having an endless supply of paper!
Some slates even have a little wooden frame around them to make them easier to hold and carry around. So if you wanted to study or practice writing while you're out and about, you could just grab your slate and go.