Monotyping is like making a special kind of painting. Imagine you have a piece of paper and you want to make a picture of a tree. Instead of just using paint and brushes like you normally would, you use a special kind of ink. This ink is very smooth and oily, and it can't mix with water like normal paint can.
So you take the ink and spread it out on a flat surface, like a big plastic sheet. Then you take your piece of paper and press it onto the ink. You have to be very careful not to move the paper around or it won't work right. When you lift the paper back up, you'll see that some of the ink has stuck to the paper and made a cool design.
But the thing is, you can only make one picture like this. Because all the ink has stuck to the paper, there's nothing left on the plastic sheet. So the next picture you make will be totally different. That's why it's called a "mono" type - it's one of a kind!
Sometimes artists will use different colors of ink, or add special textures to the ink, to make their pictures even more interesting. And when they're done, they can frame the picture and put it up on the wall for everyone to see.