An inventory is a list of all furniture, fittings and other equipment in a property provided by the landlord.
At the beginning of the tenancy, you will be asked to sign this document. You should ensure that it contains information not only about the contents of the house but the condition of everything in the house. This can be used as evidence in any dispute over damage or cleaning when you complete your tenancy period and it is time for your deposit to be returned.
Photographs taken at the same time can also be very useful and make a list of contents and note the condition.
You can find a detailed blank inventory in the Accommodation Handbook Accommodation Guide