1. I understand this proverb in the way that the friend is the only kind of person, that can always understand you. So when some people spit in your eye, your friend will give you a hand and help you, whatever happens.
2. This proverb is about the second aspect of friendship. It tell us, that friends should not only be with you and help you, they always should point us on our disadvantages. And we should listen to them, because friends are the most important thing in our life