It's simple enough, just follow the rules of the movie and accept it as gospel.
1. You can't change your past.
2. Removing a stone from the past creates a branching timeline that could be catastrophic for the people in that timeline.
3. Returning the stone to the exact point that it was removed should "trim the branch."
I might have missed some of the finer points, but that's basically how I understood it in one viewing.
My wife, who has
not seen all of the films in the series, understood (arguably) more of the plot than some people in this thread.