When first joining DRC, all new member start on the Green Tier. On the Green Tier, members receive Surprise & Delight Cashback (such as the $10 Welcome Reward and a Birthday Reward). However, in order to start earning and accruing Cashback on transactions, they first need to reach the Gold Tier by transacting 3+ times totaling to over $600 in a 12 month window. Once on the Gold Tier, members will be able to earn.
To prevent being downgraded back to the Green Tier, when members are upgraded to Gold their 12-month window resets. Within that 12-month period, members must again transact 3+ times totaling to over $600 in order to remain on the Gold Tier for the following 12-month period. The cycle continues.
You can read more about the DRC Tier levels here.