Culture is one of the more advanced Game Concepts in Civilization VI, which contributes to what makes your Civilization grow. Culture is accumulated per turn, as dependent on all sorts of things throughout the game, Buildings, Wonders, Great People, as well as Tile Improvements, Natural Wonders and a few other things. Various Civics are researched when enough culture is accumulated, depending on the Era you are in, or the Civic you are researching.
Social Policies in previous versions of Civ are now replaced with a "Civics tree", which in essence belong to their own separate tech tree and utilize Culture. Each of the Civics you unlock give you a new card to slot into your "Deck". Throughout the game you gain Civics Cards which you can slot in and out of your deck for specialization purposes. Your Deck layout changes based on the government type you pick.
Advancing throughout the Civics tree allows you to unlock new forms of government and opens new Civics cards which you can slot in and out and you can have in your "Deck" for specialization purposes, which depending on the government type will change the deck layout. Like with Technology, each individual Civic has their own unique in-game "side-quest" called Inspiration! moments, and like Eureka! moments some of these you'll get through playing the game regularly, and some will require some extra effort. Advancing throughout the Civics tree allows you to unlock new forms of government as well as unlocking Policy and Civics cards.
Advancing throughout the Civics tree allows you to unlock new Policy Cards you can have in your "Deck" and throughout the game you gain Policy Cards which you can slot in and out of your deck for specialization purposes. The Policies you unlock give you a new card to slot into your "Deck". Policies come in four variations: Military Policies, Economy Policies, Diplomacy Policies, and Wildcard Policies. Note that you can change your policy cards for free every time you unlock a Civic, or with Gold whenever you want.
Advancing throughout the Civics tree allows you to unlock new forms of Government. In addition to providing certain built-in bonuses, each government has a different configuration of slots for Policies. For example, one government might have more "Military" slots, whereas another might not have slots for Military Policies at all and instead focus on Economy and Diplomacy. As you begin the game, you start off as a Chiefdom which is your Government type. (One Slot Military, One Slot Economy). Govt. types include Oligarchy, Autocracy, Classical Republic.