Interest on your interest
With simple interest you earn only on your original deposit. With compound interest you also earn on the interest you've already received, so growth speeds up over time. That's why the green area in the chart keeps getting bigger.
The rate starts at today's 3-month Treasury bill yield, a good guide to what high-yield savings accounts and money-market funds pay right now. More frequent compounding (daily vs annually) raises your APY slightly, but time and how much you deposit matter far more.