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The Vega
Vega is a measure of the impact of changes in the underlying volatility on the option price. Specifically, the vega expresses the change in the price of the option for every 1% change in underlying volatility.
Options tend to be more expensive when volatility is higher. Thus, whenever volatility goes up, the price of the option goes up and when volatility drops, the price of the option will also fall. Therefore, when calculating the new option price due to volatility changes, we add the vega when volatility goes up but subtract it when the volatility falls.
Example
A stock XYZ is trading at $46 in May and a JUN 50 call is selling for $2. Let's assume that the vega of the option is 0.15 and that the underlying volatility is 25%.
If the underlying volatility increased by 1% to 26%, then the price of the option should rise to $2 + 0.15 = $2.15.
However, if the volatility had gone down by 2% to 23% instead, then the option price should drop to $2 - (2 x 0.15) = $1.70