WOT = wide open throttle/full throttle
The servo horn is the piece that mounts on the servo output shaft. When installing a new servo, you install the servo, power up the system, set the servo trim to zero, then you align the tires (or throttle linkage, for nitro engines), then put the servo horn on the output shaft. If you put the servo horn on the output shaft before powering it up/centering it the first time, you run the risk of the servo not being near home and causing damage when it is turned on.