You reached the maximum multiplier in an Overtake event.
For this achievement, you'll need to reach the maximum multiplier of 10 in an overtake event. I recommend doing this on the first promo event that you are offered as it is quite easy.
Every time you overtake a car, you will be rewarded with a multiplier. If you can too long to overtake a car, you will lose a multiplier. If you collide with another car or the track, you will go all the way back down to the minimum.
To get this achievement, you'll want to take your time overtaking cars, making sure not to touch anything with your car. Don't worry about speed too much, it's not as important as clean racing. Once you reach the maximum multiplier (1,000 pts in the event specified), you will unlock the achievement.