Every Logisim project is automatically a library that can be loaded into other Logisim projects: Just save it into a file and then load the library within another project. All of the circuits defined in the first project will then be available as subcircuits for the second. This feature allows you to reuse common components across projects and to share favorite components with your friends (or students).
Each project has a designated "main circuit," which can be changed to refer to the current circuit via the Set As Main Circuit option in the Project menu. The only significance of this is that the main circuit is the one that is displayed when you first open the project. The default name of the circuit in a newly created file ("main") has no significance at all, and you can feel free to delete or rename that circuit.
With a loaded Logisim library, you are allowed to view circuits and manipulate their states, but Logisim will prevent you from altering the circuits' design and other data stored within the file.
If you want to alter a circuit in a loaded Logisim library, then you need to open it separately within Logisim. As soon as you save it, the other project should automatically load the modified version immediately; but if it does not, you can right-click the library folder in the explorer pane and select Reload Library.
Next: User's Guide.