What does Genesis 31:49 mean?

Jacob and Laban and their kinsmen have heaped up a pile of stones to serve as a permanent witness to the covenant they're about to make together. This formally separates the two camps, leaving no obligations from one to the other. Both have given the place a name which means "cairn of stones" or "cairn of witness."

Now Laban also gives the place a second name, or possibly refers to an existing name for this location. Mizpah is a Hebrew word meaning "watchtower." Laban indicated it as a prayer of sorts that the Lord would watch the behavior of both parties to ensure they stick to the agreement once they are out of sight of each other.

This stipulation is almost comical, coming from Laban. The entire reason Jacob left, taking his wives, children, and flocks, is that Laban has repeatedly cheated, lied, and manipulated him. For some twenty years, Jacob and his family—including Laban's own daughters (Genesis 31:14–16)—have suffered for the sake of Laban's greed. But now, Laban invokes God as a witness, as if Jacob is the one who need to be monitored and held in check.