http://www.planningportal.gov.uk/planning/applications/howtoapply/permissiontypes#Outlineplanningconsent
Applications for outline planning permission seek to establish whether the scale and nature of a proposed development would be acceptable to the local planning authority, before a fully detailed proposal is put forward.
In essence, you put forward a general outline of what you have in mind and they decide whether that fits with policies etc. In the case of bit of land being sold, it means they can get outline permission for (say) a house of a general size and style - thus assuring the prospective purchaser that they can build <something>. The purchaser would then need to work out the details and put in for approval of them.