Modules are where you add functionalities to FreeSWITCH.
Is not very easy to write or modify a module, but neither is rocket science (by the way, as in real life, it always depends on what kind of rocket you want to build). And you can always find professional help by contacting the core FreeSWITCH developers.
This chapter will lay down all of the basic techniques needed to develop a new module:
Reading XML configuration
Adding a dialplan application
Adding an API command
Adding an event hook
Adding a channel state change hook
Firing an event