The volatility of a security measures its relative fluctuation range and is an indicator of the market risk of the security. The higher a security's volatility, the riskier it is. Volatility is usually measured over a specified period (e.g. 30 days, 60 days, 200 days). Volatility of 20 %, for example, means that the price of a security ranges between 80 % and 120 % of the current price during the period in question.
Note that volatility refers to historical prices and cannot simply be extrapolated into the future. Therefore, the volatility of options (implicit volatility) is frequently used to assess the future fluctuation range.