You cut your finger. Your sister wants you to put some medicine on it to make sure that it doesn't get infected, but you don't think that's necessary because you only cut yourself a little bit. You say:
No, it's just a scratch.
"A scratch" is a light cut. You might get "a scratch" if you bump into the sharp corner of a doorway, for example.
When you get hurt or cut, you usually check to see if the cut is deep. If it's not, you can use the phrase "It's just a scratch".
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