To put it simply, all follow up systems are there to "tell" the control system where the actuator is. For example, if there were no follow up system in the flap power drive system, the flaps would simply drive themselves to the down limit stops every time the selector was moved away from the up position. A follow up system "reports" the effects of selection to the power system, telling it when to stop motion.
Hence, a follow up linkage is some part of this feed back mechanism.