What does the phrase "Stay Hungry. Stay Foolish" mean?

Solution 1:

Stay Hungry.

Never be satisfied, and always push yourself.

Stay Foolish.

Do (or be willing to keep trying) the things people say cannot be done.

Solution 2:

Stay hungry can be interpreted as "stay eager", and stay foolish can be interpreted as "be ready to try new things" or "be ready to step out of your comfort zone".