Volatility is typically quantified using something like the standard deviation of price changes over a given period. A larger figure indicates bigger price swings, while a smaller figure indicates that the price has been moving in a more stable range.
How it is expressed
Besides volatility for individual securities, there are also indices that track volatility for the market as a whole, based on a broad market index. Periods of high volatility are often described as times when market participants' anxiety is elevated.
Points to keep in mind
Volatility is ultimately a statistical measure calculated from past price movements — it does not guarantee the size or direction of future price changes. It's also worth remembering that high volatility includes the possibility of both a large increase and a large decrease in price.