Beta measures the volatility of a stock in relation to the market. On this site we use the S&P 500 index to represent the market.
The beta of the stock is calculated using regression analysis. The S&P 500 is considered to have a Beta of 1. So if the stock beta is greater than 1 this means the stock is more volatile than the market, and if the stock beta is less that 1 then the stock is less volatile. For example, if a stock has a beta of 1.5, a 1% change in the market will result in an estimated 1.5% change in the stock. Stocks with high beta are considered to be more risky than those with low beta.
Beta = Covariance(stock returns, market returns) / Variance(market returns)
The default setting is 200 daily bars but 36 monthly bars is also commonly used for Beta.