If you scratch yourself, you rub your fingernails against your skin because it is itching.

If a sharp object scratches someone or something, it makes small shallow cuts on their skin or surface.

Scratches on someone or something are small shallow cuts.

If you do something from scratch, you do it without making use of anything that has been done before.

If you say that someone is scratching their head, you mean that they are thinking hard and trying to solve a problem or puzzle.

If you only scratch the surface of a subject or problem, you find out or do a small amount, but not enough to understand or solve it.

If you say that someone or something is not up to scratch, you mean that they are not good enough.