Wise men are people who are wise because of their experience. They know a lot about life. This quote is saying that three things that wise men fear are:
1. The sea in a storm: This means that when the weather is bad and the ocean is very turbulent and dangerous, a wise man will be scared of it because they know the potential danger.
2. A night with no moon: This means that when the night is dark and there is no moon in the sky, a wise man will be afraid because they know it can be difficult to navigate in the dark.
3. The anger of a gentle man: This means that even though the person is usually a gentle or kind person, if they get angry, it can be terrifying for a wise man, because they know that sometimes even kind people can be dangerous when they do get angry.