Legal reform in your country creates different political, economic, and social conditions for your Pops, which over time changes the fabric of your society. This change can be slow and incremental, or fast and revolutionary, sometimes literally.
There are three major categories of Laws with seven sub-categories in each, which themselves contain up to half a dozen specific Law options. As always everything in the dev diary is being heavily iterated upon, including these sub-categories, the laws you see at release will not exactly match what is being told now.
These Laws determine who is in control of different aspects of your country. It includes fundamental Governance Principles such as Monarchy and the Parliamentary Republic, which determine who your Head of State is and what kind of powers they wield.
This set of Laws define where your treasury’s money comes from and how it can be spent. Your Economic System is crucial - this governs whether your country operates on principles of Mercantilism, Isolationism, or Free Trade, among others. Income Tax determines which Pops should be taxed and what range of tax burden is appropriate. No Income Tax at all is of course an option, and legislation to such effect will, obviously, make some Pops both rich and happy.
Enshrining the rights of the individual was a hallmark of the era. These Laws define how your Pops are treated and what manner of control you can enforce over their lives. Free Speech determines the degree of control you can enforce over your Interest Groups but restrictive rights throttle the spread of innovation. The Labor Rights Laws include outlawing serfdom but extend all the way to establishing a Workplace Safety Institution to reduce the number of people literally crushed in the jaws of industry.
Laws are almost always completely independent from one another. You can create a Constitutional Monarchy with hereditary succession but Universal Suffrage, or the Autocratic Presidential Republic with a strong leader at the top of the food chain. You can have a Secret Police and still permit fully Protected Speech.