An Operation Pass is a premium feature that can be purchased with real money at any point of an Operation Season. With an Operation Pass, in addition to free rewards, you gain access to premium rewards of higher value unavailable without a pass.
One Operation Pass applies only to a specific Operation. If you'd like to get premium rewards from different Operations, you will need to unlock Operation Pass for each of them;
You can buy Operation Pass at any moment as long as an Operation Season is running, and you will receive all milestone rewards for already completed levels, which were locked before the Pass purchase.