Baldur's Gate 3 is split into three separate acts. Finishing each one progresses the story in such a way that some questlines and character interactions will become unavailable after that point. There are so many brilliant and important things to discover throughout the game and players can easily spend hundreds of hours seeing it all. However, as a result, it can also be hard to know which questlines continue into the next act and which need to be completed before moving on.
Source：https://www.cbr.com/baldurs-gate-3-complete-before-finishing-act-2/
