Laws were explained in the recent dev diary of Victoria 3. However, due to their presence in almost every other aspect of the game, it would be wise to check out in what way and how much they influence the world.
Laws, in Victoria 3, are almost always completely independent from one another. You can create a Constitutional Monarchy with hereditary succession but Universal Suffrage, or an Autocratic Presidential Republic with a strongman leader at the top of the food chain. You can have a Secret Police and still permit fully Protected Speech.
The aim is to set all countries up with the best fitting Laws compared to what they actually had in 1836. This will vary wildly between countries and greatly influence what sorts of conditions and strategies are available to you at the start of the game. For example, the USA starts with Total Separation of Church and State, ensuring no Pops suffer legal discrimination on account of their religion, while Sardinia-Piedmont doesn’t take kindly to non-Catholic Pops.
This will affect Pops who live in the country currently and limit which Pops might migrate there - few Pops would make it their preference to move to a country where they’re mistreated by law. As a result of these starting Laws Sardinia-Piedmont might have to look towards colonization or conquest if they start to run out of their native workforce, while North America is likely to get regular migration waves to help expand the frontier.
If the players do not influence Laws in any way, they can expect Pops to behave according to historical data. However, these starting Laws are far from set in stone! You might want to reform your Laws to better suit the direction your society is going.
Another reason to change Laws is that your people demand it. As we touched on in the previous dev diary, Interest Groups have Ideologies that lead them to favor some Laws over others. Reforming your current Laws to work more under your powerful Interest Groups’ Ideologies is a quick way to win their Approval, while the inverse is true as well.
Enacting a Law is far from an instantaneous, one-click affair. First off, any reform must be supported by at least one Interest Group in your government who can champion the change. Once the reform has begun it can be a smooth process that’s over in a matter of months, or it can take years of grueling debate in parliament or horsetrading between Interest Groups to pass, so make sure to plan ahead.