"Do not move the ancient boundary or go into the fields of the fatherless, for their Redeemer is strong; He will plead their case against you"
These verses are a couplet which describes wisdom toward those who are in desperate circumstances that would be easy to take advantage of. Be careful that you treat people in these dire situations with dignity and that you do not take advantage of them just because you can for God will take up their case and He will not be turned away.
This is the Hebrew word olam which means long duration, antiquity, ancient, even eternal. It is the word used to describe the eternalness of God.
This is the Hebrew word gebul which means border, boundary, territory. This proverb is about the process of taking someone's land just because you can; just because they are in a difficult position. In Hebrew society each tribe and each family within that tribe was assigned a territory and plot of land. That land was the place you farmed, the place where your prosperity and homestead was. This proverb is about taking advantage of people because of their situation. They were open to selling their ancestral lands, the father had been killed, and there was no one to work the fields. Solomon is saying that you should avoid taking advantage of people because they are in a disadvantaged situation. In other words, do not take advantage of people who are desperate. Just because they agree to a deal doesn't make it a righteous deal if their agreement is because they are fatherless or in some other way distressed.
God will hold it against you that you took advantage of these people when you could have helped them.
I can remember the case of one real estate tycoon who developed his real estate empire by getting people to sign agreements which were completely in the tycoon's favor. People were basically defrauded out of their homes for next-to-nothing because they had to sell within a short time period. It was not long after this took place that the tycoon developed incurable cancer and over the next five years wasted away in the midst of his empire.