The law means the law that was given to Moses by God on Mt. Sinai following the Israelites' escape from Egypt. Think of that: It's the only system of laws in the history of the world which was personally handed to a human being (in this case, Moses) by the Creator of the universe! God knew precisely what was required of that people at that time and gave a perfect system of laws they could live in harmony with each other and become one with Him. Some of these laws might seem archaic, harsh or petty, for example the law about divorce, in Deuteronomy 24:1. Christ explains that this was meant for those people because of the hardness of their heart and clarifies this in Matthew 19:4-5.
