APY is the acronym for Annual Percentage Yield. This measures the real rate of return on your principal token amount by taking into account the effect of compound interest. In the case of Oilz.finance, your $OILZ tokens represent your principal, the compound interest is added periodically on each Epoch (every 15 minutes). Your new principal amount is your then current OILZ token amount, plus your new rebase token amount. This total amount is what gets calculated for your next rebase rewards.
The interesting fact about APY is that your balance will grow over time! For example, with the compound interest of 0,0243%, if you start your balance of one OILZ token on the day one, your balance will be 0,0243% OILZ tokens.