Reward index is a score parameter applicable to MOONers who have at least 1 NFT Card. This parameter will depend on the number of NFT cards and types. This parameter helps users earn more on each transaction; the higher the index, the higher the reward. Check here for more info.
For example if Alex holds 2 NFT cards at the Bronze level, his reward index parameter is 6. If he owns 4 or more Bronze cards, his reward index will still be 8. If he manages to get 1 more silver card, his index will be increased to 8 + 10 = 18. The maximum reward index for an account will be 8+16+29+47 = 100.