A dummy is a unit that (generally) has no model, and the locust ability. This means that the unit is entirely invisible to all players and cannot be selected either. Because of this combination, creating dummy units with triggers and making them cast spells allows for creating spells that would be otherwise impossible to create, while giving the illusion that the original caster is the only one involved.
So "creating a dummy unit to storm bolt your enemies" is referring to something like this:
When the unit casts your shadowbolt spell, detect all enemies that are in the range of it.
For each unit within range, create a dummy unit and have that dummy unit fire a single missile at that enemy. Don't forget to remove the dummy unit after it's done casting the spell, either.