It is the train's running number, also displayed on the front of the train.
The Working Time Table (WTT) defines the journeys that 'train 205' will make in a day. A train will be assigned the number 205 and will make (all things being well) the trips set out in the WTT for train 205. Train Operators will also be assigned to trains on the basis of these numbers, and signallers, line controllers and machines use the running numbers to correctly route the trains according to the timetbale.
Just to add to what tut has said, from what I remember, the front and rear carriages of S stock will display the train number on their side panel in addition to their usual information when neither cab is active so that a relieving driver can spot which train they are meant to be taking more easily.
"I don't make mistakes. I make prophecies which immediately turn out to be wrong." "And now, excuse me while I interrupt myself" - Murray Walker OBE
Student of Mechanical Engineering at the University of Nottingham