Technically, Vega is a measurement of how much a 1% change in the volatility of the underlying asset affects the time value of an option based on that asset. Let’s say a stock trades at $74 in March and an April $75 call is priced at $3. Let’s further say that the Vega of the option is $0.10 and that the stock’s volatility is 20%.
If the underlying stock’s volatility were to increase 1% to 21%, then the price of the option should increase to $3 + $0.10 = $3.10. On the other hand, if the volatility had reduced by 1% to 19%, then the the option price would drop to $3 – $0.10 = $2.90.
A simpler way of understanding Vega is that this is the mechanism by which the market makers are able to inflate or deflate the value of time.