What was the importance of bronze mirrors in ancient Japan?

I am researching some things in ancient Japan, and during reading, I have met with "bronze mirrors" a couple of times. I tried to find websites to help me, but I can not find an answer to my question. All I want to know is why were so important, and perhaps some stories with them. If you know anything, please respond. Thank you!

He said in myth Japanese, though one could see themselves in a rusty (with the green stuff) bronze plaque, which could be declared equipped with an incomparable beauty and honor.

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