Plain meaning
Start with the shortest useful explanation before going deeper.
The annualized interest rate earned by depositors (lenders) in a lending protocol. Supply APY equals the borrow APY multiplied by the utilization rate, minus any protocol fees. For example, if borrow APY is 10% and utilization is 80% with a 10% protocol cut, supply APY is approximately 7.2%. Depositors earn yield passively as borrowers pay interest on their loans.