Every recession in the last sixty years was preceded by an inverted yield curve. Not every inversion led to a recession — but every recession started with one.
Bonds pay interest. Different bonds — different maturities — pay different rates. Plot all those rates on a chart, from shortest maturity on the left to longest on the right, and you get the yield curve. In a healthy economy, it slopes upward: longer bonds pay more, because you're locking your money up longer.
— Educational, not financial advice.