What is Divinity: Original Sin 2?
Ultimately, D:OS2 is a turn-based strategy RPG. What this means is that combat is taken in turns on a map rendered in the environment you are in. This means that combat takes place on boats, forests, towns, dungeons, towers, etc. The player controls 1-4 characters as they go through the main story. The game is done in a top down view rather than first person or third person.
Outside of combat, everything is dealt with in real time. You are free to talk to NPCs, steal items, venture off the beaten path, kill animals, or burn down everything in your path that gives you the stink eye. If you do something that would initiate combat, enemies that are engaged are then in a turn-based environment until combat is resolved. Those not engaged are free to move around and do as they like.
A character gains experience through various events like discovering new locations, defeating enemies in combat, completing quests, successfully persuading an enemy to a peaceful resolution, etc. Enough experience allows a character to level up and increase their stats in various ways.