When the ratio goes up, gold goes up more than silver, which means fear is growing. In that environment, the stock market declines. Conversely, when the gold/silver ratio declines, silver is stronger than gold, which means fear is going away and the risk-on trade is prevalent.
Another way to look at it is: when stock markets plunge, silver won't do well.
|Chart 1: Gold/Silver Ratio Vs. S&P|
You can monitor the Gold/Silver ratio here: