A Halt was a location where a train did not have to stop as a matter of course. The guard would advise the Driver of anyone travelling there, and the Driver need not stop if there was no-one on the platform.
The Halt was also an unmanned location with no staff to sell tickets or deal with trains. Parliamentary trains were also not required to stop at these locations (unless there were passengers there) because they were not classed as stations.
Going back some years, a (passenger) station was defined as such by the fact of being able to issue tickets, and a (goods) station by virtue of being able to receive and handover merchandise/goods, etc to the public.
Those old enough may remember the phrase of a station being demoted to a "Halt" during the modernisation and structural changes of the 1950s/60s/70s.
With regards to topics on fare evasion, it should also be remembered that a passenger on a train without a ticket could legitimately be there without first having bought a ticket when starting their journay at a Halt. This would not be the case where the passenger had boarded at a station.