The world is made of "stuff". This wonderful word refers to everything around you that you can see, touch feel- like a desk, an elephant, a raindrop or a pencil. However, scientists prefer to call stuff, matter.
What is it about things that make them matter? Matter is anything that you can pick up and hold, and to do that it must have weight and occupy space. Clearly, a desk, a car, raindrops, a strand of hair, and a potato all have weight and take up space. Even if the weight and space are very small. All these things are matter.
What isn't matter then? Shadows, time, light, sound, a rainbow or thoughts don't have mass or occupy space- you can't hold them. So none of these things is considered matter.