The volatility of a stock is a measurement of the amount of change of variance in the price of a stock over a specific time-period. It is common to compare the volatility of a stock to another stock to get a feel for which may have less risk, or to a market index to compare the stock's volatility to the overall market. Generally, the higher the volatility, the riskier the investment is in that stock.
Volatility is calculated by taking a rolling-window standard deviation on the percentage change in a stock (and scaling it relative to the size of the window). The size of the window affects the overall result. The wider a window, the less representative the measurement will become. As the window narrows, the result approaches the standard deviation. So, it is a bit of an art to pick the proper window size, based on the data sampling frequency...