The Reward Track allows you to earn rewards by Raiding and completing Daily Challenges throughout the season. In the Reward Track you can earn things such as currencies, Raid Enhancements, cosmetics (suit skins, weapon skins, and decals), and a mystery track reward! 

When an Expedition season ends, any rewards that you have not earned will be added to the “Randomized Rewards” of future seasons. You must first unlock all the rewards from the current season before rewards from previous seasons are added to the pool of randomized rewards.