What is the APY?

APY stands for Annual Percentage Yield. This measures the real rate of return on the amount of your principal tokens by taking into account the effect of compounding interest. In the case of DiamodLock, your $DML tokens represent your principal, and the compound interest is added periodically on every Rebase event (Every 30 minutes).
Your new principal amount is your then-current DiamondLock token amount, plus you're new rebase token amount. This total amount is what gets calculated for your next rebase rewards.
The fact about APY is that your balance will grow not linearly but exponentially over time! Assuming a daily compound interest of 2.379%.
If you start with a balance of 1 $DML on day 1, after a year, your balance will grow to about 533,848 $DML. That is the power of compound interest!