Hello Farah,
The subsystem is a feature in OptiSystem software that you create. Double click on it and view the components inside it. You can build your project without using subsystems. You need to understand how the signals are generated and used to modulate the subcarriers in the OFDM modulator component. You may double click on each component and use the help button to access the datasheet of the component. You will find details on the component and its theory.
Regards,
Ahmad