Civilization 6's Governments and Policies are an entirely new addition to the Civ series, bringing with them a new level of control over the type of civilization you hope to run.
Unlocked by progressing through the all-new Civics Tree, which is done by earning Culture - much the same as Tech Tree advances come from earning Science - Governments each grant a unique Inherent Bonus and Legacy Bonus, a selection of slots for Policy cards, and points towards city-state-wooing Envoys. Policies will comes from the same method of Civic Tree progression, with each one offering various benefits to your yields, units, Production and more.
Careful management of your Government choice and Policy unlocks, then, is crucial for an efficient playthrough, particularly if you have a specific victory condition - such as a Scientific, Military, or Religious Victory - in mind.