Bend protocol uses the same interest model as AAVE's aToken. You can easily understand what bendETH is if you know AAVE's aToken well. The core value of bendETH and aToken are the same. Reference
The difference between bendETH and aETH is the liquidity on Uniswap.
Bend's bendETH is the interest-bearing token that is minted and burned upon deposit and withdrawal. The bendETH value is pegged to the value of the corresponding deposited asset at a 1:1 ratio and can be safely stored, transferred, or traded.
All interest collected by the bendETH reserves is distributed to bendETH holders directly by continuously increasing their wallet balance.