When you reach Chapter 2 Mission 3, you'll meet Will the Wise and unlock the ability to get rewards. The "Rewards" icon will be on the bottom right of your screen.

You will accumulate rewards even if you aren't logged into the game, but your rewards will be full after 8 hours, at which point you won't collect any more until you claim them!

The further you progress into Story missions, the greater your rewards collecting time will be, and the higher rarity your equipment from rewards will be.