Its showing the yield curve for US treasuries over the last few decades. The bottom axis is the maturity i.e. a 3 month bond, 5 year, 7 year, up until 30 years. The y axis is showing how much interest you would get from these bonds (the yield).
Longer dated bonds should have higher interest rates than shorter dated bonds because you are taking more risk by holding onto the bond for a longer time. When the line flattens out it means investors are putting their money into longer dated maturities because they're getting fearful and want their money in a safe place. This drives the yield down so that the long dated bond yields basically equal the short dated bond yields. This is usually an indication of a recession or bear market approaching. But as always, there are exceptions to the rule. In the 90's it flattened a couple times and if you look at 2006 it took 2 years for 2008 to happen.
Im probably oversimplifying it and im typing this on my phone as i walk, but you could google "inverted yield curve" and learn more.