It refers to a portfolio with 60% in stocks and 40% in bonds. It has been popular for many years, especially buy and hold investors.
Why 60/40 portfolio no longer works?
Because the yields on the bonds are heading lower and lower, so the insurance value offered by bonds are disappearing. Plus, the government are boosting economy which will bring up inflation, and as interest rates start rising, bond prices will fall, talking about triple hits to bonds holders.
What's a better portfolio?
It depends on what type of investor you are. If you are young and can tolerate volatility, stocks should carry more weight in your portfolio. If you need fixed income, you can find other types of fixed incomes, such as dividend paying stocks or convertible bonds which could offer the best of both worlds.