When the primary side is connected to the AC power supply, AC current flows in the winding, and an alternating magnetic flux with the same frequency as the applied voltage is generated in the iron core, and the alternating magnetic flux induces electromotive force with the same frequency on the primary and secondary sides respectively.
When the secondary side has electromotive force, it outputs electric energy to the load, which realizes the transmission of electric energy with different voltage levels. Because the magnitude of the induced electromotive force is proportional to the number of turns of the winding, the voltage of the secondary side can be changed by changing the number of turns of the primary and secondary sides.
Briefly: Electromagnetism: alternating current is introduced into the primary side, and a changing magnetic field is generated in the iron core.
Magnetism: changing the secondary side of magnetic field will produce induced potential on the secondary side.
Reprint: Internet