This expression means that the property, although currently occupied, is for
sale on the understanding that the present occupants will move out prior to completion
and that you will be available to move into the property immediately.
The alternative is for a property to be sold subject to an existing tenancy, when it may be sold purely as an investment opportunity, or there may be part vacant possession, with only certain rooms being occupied by a statutory tenant. In these circumstances it is important that you discuss the matter carefully with your legal adviser since landlord and tenant legislation is complex and can change from time to time with new housing legislation coming through.
Vacant possession is naturally assumed when a property is put on the market and will not be noted on the particulars but if any delay or special conditions apply the estate agent will make an appropriate note.