It's an investment philosophy developed by Harry Browne that holds equal parts stocks, gold, cash and long-term treasuries.
You can check out this forum:
http://gyroscopicinvesting.com/forum/index.phpCraig Rowland's 2012 book called The Permanent Portfolio is probably the best starting point. The basic idea is that different asset classes perform well in different economic environments. In the 1980s & 1990s, stocks and bonds did great. In the 1970s & 2000s gold was far and away the best performer. A lot of people don't like it because they don't like one of the assets classes. For me at present it's long-term treasuries. But the basic idea is that it's impossible to time the markets, and that by rebalancing periodically you sell some of your winners and load up on other assets when they are on sale.
Anyway, I am not a pitchman for the portfolio. I just think it's a good alternative to a 60/40 stock & bond split. Others on this forum don't agree and you should weigh what they have to say as well.
For the record, I am 55 years old and I want a portfolio that protects my principal and has real (after inflation) positive returns for all rolling five and ten-year periods over the last 40 years (a 60/40 stock split hasn't accomplished that).