To build sophisticated circuits, you will want to abstract portions of the circuit into smaller components (similar to the use of functions in programming). This abstraction allows you to design a single circuit once and then use it multiple times. Logisim provides a facility providing this abstract, called a subcircuit.
If you're familiar with computer programming, you're familiar with the subprogram concept (called subroutines, functions, or methods in different languages). The subcircuit concept is analogous to the concept in computer programming, and it is used for the same purposes: To break a large job into bite-sized pieces, to save the effort of defining the same concept multiple times, and to enable debugging.
Creating circuits
Using subcircuits
Logisim libraries
Next: Creating circuits.