I don't really know if the question is suitable to be submitted here or in the Physics StackExchange. The problem is I don't quite understand this sentence from "Practical Electronics for Inventors" Book, p.377
When a load is attached to the secondary, the secondary current sets up a magnetic field that opposes the field set up by the primary current. For the induced voltage in the primary to equal the applied voltage, the original field must be maintained. The primary must draw enough additional current to set up a field exactly equal and opposite to the field set up by the secondary current.
The explanation says that when a load is attached to the secondary, the current in the primary must change to keep the applied voltage field the same. I have no idea what that means and why does this phenomenon have to occur in the primary, when there is a load in the secondary.
Furthermore it's assumed that the magnetizing current will be very small in comparison to the current after the circuit being loaded, which I also don't understand.
I need some more in-depth explanation. I have quite good background in physics of electricity from first year of engineering college, so feel free to give a deep explanation about what is happening.