Aside from the ten commandments, you won’t find much divine lawmaking in the books of Genesis, Exodus or Numbers.
There’s the occasional suggestion (be fruitful and multiply) and the odd prohibition (don’t eat from that there tree) and stage directions (go tell him, “Let my People, go”). There are, of course, the grand cosmological explanations and an easy banter between the Patriarchs and God. Of course, there is the implied good behavior (faithfulness), but not much real law as such.
The real law begins with Leviticus, which is the largely ritual law, addressed to the Levites. These folks were members of the tribe of Levi, a priestly tribe assigned to care for the Tabernacle, discussed later.
It continues with Deuteronomy (or second law), which contains a series of discourses ascribed to Moses.
These two books form the core of the Law referred to by the Jews. Not all of it applied to everyone.
The Law covers nearly every aspect of life. Initiation of males into the community (circumcision), dietary rules (no pork, no mixing of dairy and meats, etc.), prayer, marriage, business and so on.
Modern Judaism does not literally demand the observance of all of the Law, although certain Orthodox strands of the religion attempt it. In fact, Judaism became a religion of another collection of writings, the Talmud. This very complex collection, written between the second and fifth centuries of our era, contains a wide range of rabbinical teachings, rulings and interpretations of the original, biblical Law.
Much like court rulings on the U.S. Constitution form the body of constitutional law, the Talmud’s interpretation and application of the Law is also regarded as the Law by extension.
Christianity stopped regarding most of the ordinances, particular those having to do with ritual and dietary rules, as normative for Gentiles at the urging of Paul of Tarsus, and later relieved all Christians—including Jewish followers of Jesus—from their observance.
There remain some very literal-minded Protestant readers who take some, but not all, ordinances very seriously. For example, many Evangelical Christians regard Hebrew Bible injunctions against homosexuality as in effect; however, they have no problem having bacon with their eggs in the morning.
The Law is important if you are Jewish or interested in Judaism. It is also important for Christians who wish to understand the New Testament better. It is what Jesus, and later Paul, were talking about.