Ideas in Hearts of Iron IV serve to modify various aspects of a nation's status, such as national unity or production, and in some cases serve to unlock certain capabilities. In the context of many other games, they can be generally understood as "buffs", "de-buffs", and "unlocks".
Conscription Laws translate directly to available manpower, but at later stages also begin to affect everything from construction speed to factory output.
You can find more about Conscription Laws in the corresponding section on the Official Hearts of Iron IV wiki.
Trade Laws primarily concern the import and export of resources to and from the country, though also translate to the general openness of the country, thus also affecting things like construction speed and research time.
You can find more about Trade laws in the corresponding section on the Official Hearts of Iron IV wiki.
Economy Laws directly impact the production and construction speeds of a country, and tie heavily into whether or not the country is currently in a state of war.
You can find more about Economy Laws in the corresponding section on the Official Hearts of Iron IV wiki.
A Kaiserreich specific mechanic, the Stability system can have a major impact upon all aspects of a nation. Stability is present in the form of laws which you can see in your political screen next to trade laws, and are influenced by your political power. Having under -50PP will make you a potential target to get at low stability, -250 will get you to very low stability and -500 will get you in a disaster; you want to avoid that, trust me. On the contrary you'll be able to go up to very low stability if you're above -500, to low stability if you're above -250 and to average stability if you're above -50.
The mechanic to go up to good and very good stability is a bit different, as you must get 150PP and then select the corresponding law in the stability screen. You'll then eventually lose the positive stability levels if you happen to get under 0PP.