Collection Milestones are top-end rewards you can gain from earning Collection Milestone points by upgrading your Assets. Once you have earned enough Collection Milestone points to reach a Milestone, you can then claim that Milestone’s reward!

There are various types of valuable rewards you can collect as you progress through Collection Milestones, such as: 

Legendary Drivers
Power Ups 
Drivers/Car Components

You can access your Collection Milestones track by selecting your ‘Drivers’ or ‘Car Components’ tab on your loadout screen. You can then access the track at the top of the screen.