This is when a tenant says they are living in a property let to them but instead lets all or part of it to someone else without the landlord's consent. They may have left a family member there, or may have found a private tenant of their own who may not realise they are living in a social housing property.
Additionally, the new person at the property may be paying much higher rent than the tenant had to pay, allowing the tenant to make an unlawful profit.