Vega, one of the five greeks used for measuring options risk, measures risk exposure to changes in implied volatility and tells traders how much an option's price will rise or fall as the volatility of the option varies. In other words, vega is a measure of how much the value of an option changes when volatility rises 1 percentage point.
Vega can rise or fall without a price change in the underlying asset. It can increase if the price of the underlying asset moves quickly, especially when the stock market declines fast or a commodity makes a big move. Vega falls as the expiration date of the option nears (this contrasts from the movement pattern of theta).
If an option has a vega of -3.5, then for every 1% rise in implied volatility of that particular option contract, the price of the option will decline $3.50. If the implied volatility of the above option were to decline 1%, then the price of the option would increase $3.50.