APY stands for Annual Percentage Yield. This measures the real rate of return on your principal amount by taking into account the effect of compounding interest. In the case of FP, your $FREE tokens represent your principal, and the compound interest is added periodically on every Rebase event (Every 15 minutes).
Your new principal amount is your then current $FREE tokens amount, plus your new rebase token amount. This total amount is what gets calculated for your next rebase rewards.
The Power of Compound Interest - It is important to note that your balance will grow not linearly but exponentially over time. Taking a compound interest of 0.021447% / 15 minutes.