Much like in its predecessor, Europa Universalis: Rome, the player can tailor their country to achieve particular goals by picking national ideas.
There are 36 national ideas the player can choose from. Each idea has its own bonus and belongs to one of four categories: Military, Economy, Oratory, and Religion. Some ideas are available from the start, while others are unlocked by advances during the course of the game. Selecting a new national idea costs 20 Political Influence.
The number of active national ideas depends on the country's government type. Generally speaking, tribal governments can have two national ideas, while republics and monarchies have three. The special government types Dictatorship, Empire, Imperial Cult, and Athenian Republic have four slots.
Each government in the game comes with a base bonus specific to that government. There is also a secondary bonus for each government that is given if the categories of the chosen ideas correspond to what is expected by that government form.
For example, in the screenshot Carthage is an Aristocratic Republic. This means that they need to pick two military ideas and one oratory idea to receive the additional bonus.
Etruria, which starts as an Oligarchic Republic, instead needs one civic idea and two oratory ideas to get the additional bonus.
The Government bonus of each type of government can be found in the Government Types page.