What permitted development rights are, and what is being withdrawn
Certain categories of work — some extensions, some outbuildings, some changes of use — are granted planning permission nationally by order, so no application is needed provided the work stays inside the limits. That is permitted development.
An Article 4 direction removes a specified part of that national grant for a specified area. It is made by the local authority, usually because the cumulative effect of the permitted work is causing a problem locally. Basements are a recurring example: subterranean construction in dense residential streets generates years of disruption and structural risk to neighbours, and several authorities have withdrawn the right.