Most of the confusion surrounding Interstellar stems from scenes late in the film, when astronaut Cooper enters a black hole and things start to get a little… strange. The next thing you know, Coop is floating in a crazy space library that connects to his daughter
Murphy’s bedroom in the past, and there’s something to do with a wrist watch. So what does all this actually mean?
Well, what we’re dealing with here is a time paradox. The “space library” is in reality an interface designed by future humanity to provide their ancestors with the means for inter-temporal communication. Basically, it lets Coop send messages to Murphy in the past, by interacting with the area surrounding her bookcase (hence the library-like interface).
As for the watch, it represents another instance of Coop manipulating the gravitational forces in Murphy’s room, in order to share with her – via a simple numerical code – the quantum data needed to solve the equation for mass interstellar travel. By doing so, he ensures the survival of humankind, thus allowing for later generations to set in motion the chain of events that allowed Coop to contact Murphy in the first place – which is where the previously mentioned paradox arises.