Hardness
Hardness is how well a solid resists being scratched or dented.
Think of it this way
A butter knife pressed into soft butter sinks right in and leaves a groove. Press that same knife on a glass jar and it leaves no mark. Hard things push back when you try to scratch them. Soft things give way.
Explaining hardness by grade level
Some things are hard to scratch. Try pressing your nail into a brick and then into soft clay. The brick does not get a mark, but the clay does. The brick is harder because it fights back against scratches.