APY stands for Annual Return. This measures the true rate on your principal by taking into account the effect of compound interest.
Your darth tokens represent your capital and compound interest is added periodically on each event (15 minutes).
One interesting strong feature about APY is that your balance will not grow linearly but exponentially over time! If you start with a $1 DARTH balance on Day 1, after a year, your balance will grow to about $3,830 DARTH. This is the power of compound interest!