The library module is that part of the IF authoring system that establishes the default behavior of objects. The plain-vanilla simulated world is pre-load with a set of rules for fundamental objects. The default library elements are, at the very least, building blocks for amusing puzzles. The more an author wants to diverge from the default behaviors specified by the library the more coding is necessary.
The library module also frees the IF author from having to worry about technical details such as allowing the user to save, load, or record a transcript in media res. The library handles abstract concepts such as the passage of time and the sensibility of objects (you can't smell or touch the rose if you put it in a closed jar, but you can can still see it).
The library might invisibly provide a few rules defining the basic behavior of any object that the IF author identifies as a "door":
1) The player-character can enter any "door" object, as long as it is in the same room and open.
2) If the player-character walks into a closed door, the game will automatically try to open the door first.
2) After successfully entering the door, the PC is transported to a location specified by the IF author.
If your story includes a magic spell that can change where the door leads, or if you want a hideous creature to leap out from behind the door, you will have to write a few lines of code.
One particular "door" might be "locked" and "concealed", because it is in fact disguised as a bookcase. If you want to create a puzzle that has the bookcase swing open when the player tries to extract a certain book from the bookcase, you would need to write a few lines of code to "unlock" the door in this manner.