Ops or Operations, are small to large challenges that can change every day or week. Some are fairly easy and can be completed in a single battle while others may require multiple battles and attempts.
Some challenges are repeatable daily and reset. Doing these once daily counts as one of your eight.
To discover your Ops there is an Ops TAB available in your garage.
For example: One Ops challenge is to damage three separate tanks in a single battle using a Tier III-Tier X tank. This can be done five times in a day. Completing the Op once in a 24 hour period or maxing it out and doing it all 5 times doesn't matter. The achievement only counts you as having completed one Op.
So, simply put, you could do the exact same Op, once a day, for eight days in a row, and get the achievement.
The achievement pops once you have completed the 8th unique (Ops are unique on a daily basis) challenge, meaning when the tank that you achieved it in is returned to the garage and the battle stats saved.