It means: try to find a stock with a growth rate approaching 15% per year (which is hard and very often expensive) and a ROE of 15%.
The stock market offers new stocks every year and some rookie stocks seduce a lot of people. Some people say "things have changed, it's not like before, we now must value stocks in a different way!".
Well, some people said that in 1999-2000, just before the mega Internet bubble bursted. And probably that some people said that in 1929 too. But I don't remember exactly. It's too far for my memory to be accurate.
My portfolio is based on that 15% rule. I try to be rational but a bit agressive. Once again, it's just my method and it may not be the best in the world. But that method implies that I buy stocks that offer a good growth, but not a crazy growth that would bring too much competition or that would be very hard to manage.
First and foremost, I understand what I do with that method. It should always be the first thing to consider: do you understand what you're doing with the savings of your life?