It is many years since I had anything to do with this, so others may correct the following.
In the rules ‘token’ is used for anything that may be removed from an instrument and issued to a driver at one point, usually one end of the section, and returned to another instrument once the train is clear of the section. As long as there is only one train in the section at a time, trains can run alternatively in either direction or follow one another.
With a ‘staff’ nowadays (again in the rules) there can only be a single one for the section and it must be returned to the signal box (or location) the train started from before another train enters the single line; effectively it is only used in dead end branches where the train goes up and then comes back. It used to be normal to operate through single lines with a staff, but that was inflexible as after one train had gone in one direction, the next had to be in the other.
Physically, a token is a bit like a lumpy, old fashioned key made out of aluminium or some similar alloy (they can be known as ‘Key
Tokens’). A tablet, which is almost extinct, functions like a token but is a flat disc or similar, usually of metal. A staff, as now used, can be any suitable object from a hefty bit of metal with ground frame release keys built in to a piece of wood a foot or so long for ease of handing over. (*)
One thing they all have in common is that they will carry the name of the section, either engraved on the metal or on a plaque - usually brass - screwed to the wood. The tokens and tablets also have various manufacturing differences which ensure that only one of the correct configuration can go into a particular instrument and one signal box never has two instrument of the same configuration. Just to make it complicated, a staff could actually be an old token, but if it functions as a staff, then that is what it is called by the operators (though probably not by the S&T when they are looking for spare parts).
* There used also to be electric staffs, which functioned in the same way as tokens (and tablets) and would be referred to as tokens by the operators. They were metal rods of varying lengths, but are no longer used by Network Rail.
I hope I haven’t confused too much, but in essence there are two similar, but slightly different, meanings depending on whether you are dealing with operating rules or equipment technicalities.