Compound Interest
Interest that is earned not just on your original deposit, but also on the interest that has already been credited to your account.
Compound interest is the engine that makes long-term saving powerful. Instead of only earning interest on your original deposit (called simple interest), you also earn interest on the interest itself.
Most HYSAs compound daily and credit interest monthly. Concretely: if you deposit $10,000 at 4.00% APY, you do not just earn $400 over the year. You earn approximately $400.81, because each day's tiny bit of interest joins the principal and starts earning its own interest the next day.
The longer the time horizon, the more compounding matters. $10,000 at 4.00% APY grows to about $14,802 over 10 years thanks to compounding — that is $802 more than simple interest would have produced.
Two compounding details to know: (1) the more frequent the compounding, the slightly higher the effective yield, but the difference between daily and monthly compounding at the same APR is small; (2) the APY already includes the compounding effect, so you do not need to do any math when comparing accounts as long as you compare APYs.