Forfeiture means the lease can be terminated and the property revert to the freeholder. This could arise arises if the leaseholder breaches the terms of the lease.
An example could be a failure by a leaseholder to maintain their flat. The law restricts the use of forfeiture even where the lease has been breached; however, if you find yourself in this situation you should seek legal advice. If an alleged breach is not admitted or agreed by the leaseholder the landlord will have to apply to the First-tier Tribunal or a court for a determination of the breach before they can start any forfeiture action.