“Stay hungry” means you must always want more and more in terms of new opportunities.
“Stay foolish…” means, when everyone thinks that you are a fool for trying to do what you are doing, then pay no attention to them, simply stay a fool, continue to do what you set out to do, and in the end, someone will be eventually laughing to the bank.
Steve knows this too well, so as Bill Gates, even Mark Zuckerberg, and so as Donald Trump (especially when he was just a small time apartment manager in Cincinnati, Ohio).
At the beginning, non-visionaries always tend to stand on the side line, laughing at whomever is trying out new things, but in the end, they come running and begging, asking for hands out.
Now, who’s a fool?