Typically, people that lend out their money for longer periods of time want a higher interest rate for lending out that money, than if they lend out at a shorter time frame.

The yield curve inverts when the interest rate you get for lending long term bonds (~10 years) is less than the interest rate you get for short term bonds (~1 year). Every recession in the last 8 or so recessions had been predated by a yield curve inversion, so people take it to be a predictor of a slowing economy now.