However, it's not a literal description of plates. It's used to describe someone who is unassuming, mild-mannered, and unlikely to cause trouble.
The closest English equivalent would be:
* Mild-mannered
* Unassuming
* Easygoing
* Non-confrontational
* Peacemaker
* Harmless
The phrase implies that the person is so gentle and kind that they wouldn't even accidentally break a plate, let alone cause any real harm.