I have an Indian made HMT automatic wrist-watch,it is written on it "21 jewels".What does it mean?

That watch does not need any battery or manually winding to run it. Its winding is done automatically merely with a few hand movements while wearing it on the wrist. Is 'JEWEL' any kind of standard or grade or unit in wrist-watch mechanism? Please describe it in detail what is a JEWEL.
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