Vega is exposure to implied-volatility. Long options have positive vega: they gain when IV rises and lose when it falls. Short options have negative vega. Longer-dated options have more vega than short-dated ones.
Vega is why buying options before earnings can lose money even when you guess direction right: the iv-crush after the report removes value faster than the move adds it.
Example: an option with vega of 0.12 is priced at $4.00 with IV at 30%. If IV drops to 25% with the stock unchanged, the option is worth about $4.00 - (5 x 0.12) = $3.40.
Related: implied-volatility, iv-crush, theta, delta