With inflation, coins with a higher metal value began to have a nominal value lower than the value of the metal they were made with, causing people to start melting these coins to obtain the metal. Thus, bad coins ended up dominating the market, and good coins disappeared.
It seems to me that you have not understood it well. Where did you get that from?
It's a simple reasoning. If a coin is made of silver, it is easily worth 1$, but the amount of silver that this coin has is worth 2$. It's worth melting down that coin and selling the silver, rather than using the coin to buy something for 1$.
But, it explains the theory of Gresham's Law better.