Hidden Move = HM
I assume you know about HMs, which are the moves that you gain the ability to use after beating a Gym Leader, and have a unique effect in the overworld. The HMs vary between Generations, though Surf, Fly, and Strength are come of the common ones. The reason you cannot Migerate a Pokemon with a HM move is because it could be last resort; you could migrate the only Pokemon you have that can learn the Hidden Move, and you might get totally stuck somewhere in game.