Gresham's law, named after Sir Thomas Gresham, who was a financial agent of the English Crown in the 16th century, states that overvalued money drives undervalued money out of circulation: "bad money drives out good" for short.
More accurately, the law should state something along the lines of "government-enforced parities that alter the market value of money have the effect that overvalued money drives out undervalued money."
Because in a free market those products and goods that are of higher value stick around longer than low-quality products, Gresham's law only applies when government monetary intervention is present.