The term 'unfurnished' is simply used to describe a property in the rental market that is let without furniture. Exactly what an unfurnished house means in practise can vary, from an empty shell without light fittings, to a property with a full kitchen, and basic furnishings but no beds and sofas.
The advantages of unfurnished lets are many. To the landlord, there is a lower up front capital cost, and lower ongoing cost and hassle with maintenance, as well as fewer tax debates around wear and tear. To the tenant, while there may be greater costs of acquiring furniture and moving it in, they are able to feel like their house is customised to their own tastes.
Typically, someone looking to rent an unfurnished property will already have their own furniture, perhaps because they have rented for a long time, or recently sold and are renting while looking to buy again.