Autothrottle's control target during autoland
The auto-throttle system of a typical airliner (like e.g. the Boeing 737) will maintain the approach speed (usually $ v_\mathrm{ref} + 5 \, \mathrm{kt} $, depends on the wind) during final approach. It should not reduce thrust before starting the flare because the speed could decrease too much, which could in the worst case result in a stall.
The Boeing 737 FCOM (v2 4.20.19 Automatic Flight - System Description, see also this answer for a full description of the autoland) says:

The A/P flare maneuver starts at approximately 50 feet RA and is completed at touchdown. [...]
The A/T begins retarding thrust at approximately 27 feet RA so as to reach idle at touchdown. A/T FMA annunciates RETARD.

So during the flare the thrust is decreased such that the thrust levers are closed at touchdown. This (together with the increase in pitch) will start to reduce airspeed. Ideally, the aircraft then touches down at or about reference speed $ v_\mathrm{ref} $. The thrust reduction should be smooth because the thrust-pitch-coupling of engines mounted under the wing will cause a pitch down moment, which needs to be counteracted with control inputs.