Volatility trading is the term used to describe trading the volatility of the price of an underlying instrument rather than the price itself. Volatility trading is simply buying and selling the expected future volatility of the instrument. There are many ways or rather innumerable ways to measure volatility one of the easiest is - a range of the price bars.
Volatility is often thought of as measuring risk or uncertainty. We are never sure where an asset is going to be at some point in the future, but a more volatile asset or underlying will have a wider spread of likely ending values, compared to a less volatile asset for the same time period.