This is the third expansion to Europa Universalis III, released after the compilation know as Europa Universalis III: Complete. As the others games of the series, you are a power behind the throne in a open end universe waiting for you to expand, conquer, trade, make policies regarding all kinds of internal affairs, converting the people to the state religion (or converting the state to people's religion), signing treaties and a lot of other options.
The focus now is the struggle in diplomacy and internal affairs. The new casus belli system understands the differences of the motivations of war and act accordingly. Each different casus belli has a different effect on peace resolutions as give more prestige, less infamy and requires lower score to receive demands legitimated by that casus belli. And, accordingly to the situation, they may aggravate your situation in a defeat too due the sheer loss of prestige.
The other focus is the succession, as now, if a king dies without a heir (or a too young one), the following regency period could be disastrous for the kingdom, also rulers with low legitimacy may face strong opposition from pretenders to the throne. Some other minors details were changed, like he election of the pope, and the power of the Holy Roman Empire emperor.