The Cease Charge is applied if you cancel your broadband and do not use an approved method to move to another provider.
It is set out in the Terms and Conditions you agreed to when you signed up.
If you use this link and click on "What is the Cease Charge" it explains more.
As has been posted already the cease charge and the explanation of when it will be charge are covered in th T&C which you have already agreed unless like most you just just ticked the box to sappy you agree without reading them
it is not a hidden charge