There's nothing special about a Logisim library. If you have defined a project with several circuits, you can save it into a file and then load the library within another project. All your several circuits will now be available. This feature allows you to reuse common components across projects and to share favorite components with your friends (or students).
You cannot change a loaded library, although you can view its circuits. If you change the library file, you will need to save and re-open your file for the changes to take effect.
Each file has a "main circuit." This has no significance beyond designating the first circuit viewed in the canvas when you open the file. The name main also has no significance; you can feel free to delete or rename that circuit.
Next: User's Guide.