Volatility is a measure for variation of price of a financial instrument over time (here the price of one Bitcoin in USD). It allows to understand how much Bitcoin price varies over time and compare it to others currencies.
Note: the Volatility of Bitcoin is not fully comparable with others currencies because Bitcoin exchanges markets never closes.
The volatility is the standard deviation of returns (log of the ratio between the initial price and the final price over a period). The calculation here is done with 1-day returns.