This quote comes in chapter 1. Okoye has come to collect his debt of two hundred cowries from his friend, Unoka. As he tells Unoka the purpose for his visit, Unoka begins to laugh.
"Look at that wall," he said, pointing at the far wall of his hut, which was rubbed with red earth so that it shone. "Look at those lines of chalk" and Okoye saw groups of short perpendicular lines drawn in chalk. There were five groups, and the smallest group had ten lines. Unoka had a sense of the dramatic and so he allowed a pause, in which he took a pinch of snuff and sneezed noisily, and then he continued: "Each group there represents a debt to someone, and each stroke is one hundred cowries. You see, I owe that man a thousand cowries. But he has not come to wake me up in the morning for it. I shall pay you, but not today. Our elders say that the sun will shine on those who stand before it shines on those who kneel under them. I shall pay my big debts first."
So, in this case, those who have given Unoka more are the ones standing. Those who have given little are the ones kneeling. Unoka will pay those he owes more first, because they are the more important than the small debts. Unoka is also making fun of Okoye in his own way. He is suggesting that if a man to whom he owes 1,000 cowries isn't coming around asking for his money, Okoye shouldn't be either.
This proverb is important because Okonkwo seeks to be one of the men standing, not kneeling. He despises his father & his debts, seeing him as weak and feminine. Okonkwo never borrows more than he can pay back, & it is because of his father that he attempts to masculine in all areas.