What is the meaning of "Posh'im be'Nafshosam"?
Rashi: It means that they were sinners in that they by quarreling with Hashem, they forfeited their lives.
What are "Riku'ei Pachim"?
What sort of sign did the copper plates display?
Rashi: They served as a reminder that they belonged to the people who had quarreled against the Kehunah and had been burned.