cTokens and aTokens provide a mechanism to accrue interest just by holding a digital asset. These tokens are minted and redeemed when users deposit or withdraw a digital asset in Compound, Aave or other similar protocols. As the money market accrues interest, which is a function of borrowing demand, cTokens / aTokens become convertible into an increasing amount of the underlying digital asset. In this way, earning interest is as simple as holding a a token. These tokens effectively represent your balance, including interest.