In simple terms, jailbreaking allows you root access to your device, allowing you to see the system files, much like My Computer for Windows, or Finder for Mac, where you can see all the individual files, down and dirty. It also allows for you to run unauthorized codes/commands that Apple does not normally allow for, which in turn allows for the modification of your device's software components.
I've moved your post to it's own thread as it was hijacking another thread, which is against our rules. Being on it's own will also give it more attention.