Originally the rental value of the property, but this long ago became a notional value and since Council Tax has replace Rates the ratetable value is now only used for Water rates if you don't have a meter.
Rateable value refers to a theoretical rent for your property and was set by the council many years ago.
Generally, the bigger the house, the higher the RV so water is based on this so bigger houses pay more.
However, most people opt for a water meter these days.
Hope that helps.