As you build circuits that are more and more sophisticated, you will want to build smaller circuits that you can use multiple times as a module nested within larger circuits. In Logisim, such a smaller circuit that is used in a larger circuit is 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 facilitate debugging.
Next: Creating circuits.