Asphalt milestones are tiered rewards earned by accumulating points through ranked races. With each race, you will be increasing your Series Score and unlocking new rewards every time you reach a milestone. Finishing 1st will reward you with more points and the higher your league is, the more Series Points you earn from each race. 
Kindly note that to receive additional rewards for some milestones, you need to have the Multiplayer Bonus Pass. The extra rewards are marked with a red and white icon (you can see it above the rewards).