Imagine you get to school and someone got there earlier and said, 'that chair is mine now,' and just took your seat. Not fair, right? That's exactly what moving the boundary stones means: taking or using what belongs to someone else as if it were yours.
But there's a harder version: when you could take something from someone else and no one would ever see. That's when your real character shows! A person of integrity acts the same way whether everyone's watching or they're all alone. And you know who always sees? God. He's the greatest defender of those who have no way to protect themselves. When you protect what belongs to someone else, you're acting just like Him!