Here we are told of interlinking things not to do – “move an ancient boundary stone” and “encroach on the fields of the fatherless.” Boundary stones (see Dt 19:14; Pr 15:25; 22:28) were informal stone monuments regularly used to mark the corners of family fields. They are called “ancient” here, at the time of the proverb was written.
The first command is a general principle to not move any “boundary stone.” The implication being to adjust the size of anyone’s field, making it larger or smaller.
The second command is more specific – particularly don’t mess with “the fields of the fatherless” (orphans). Whatever the precise status of this orphaned person, the implication is they lack this mature adult guidance and support in their lives.
In the Bible world, correct identification of the full extent of a family’s all-important grain fields was critical for their survival from one year to another. These ancient stones ensured that.
Presumably, both these acts would be perpetrated for one’s own benefit, at the expense of others. These commands should have been sufficient guidance for civilized people. But the next verse (:11) offers additional incentive!
Those who own these fields and do not see what is going on – especially orphans – have a “Defender” who is “strong” and is going to make the perpetrator pay. He sees and He knows and He will adequately address the wrong.
While the Hebrew text does not specifically indicate that the Defender is God, the translator suggests it is, by capitalizing the word. That makes this an even more serious matter. I better not rip off my neighbors because God is watching over them and He will get me for it!