This measures the security's 50 Day Moving Average price divided by the 200 Day Moving Average price.
On a stock chart, the golden cross occurs when the 50-day MA rises sharply and crosses over the 200-day MA.
This is seen as bullish.
Work by Seung-Chan Park has shown that companies whose 50d MA is far above the 200d MA significantly outperform those companies where the 50d MA is below the 200d MA.