Step-down Transformers are used to decrease electrical voltage when required for electrical devices, equipment or machinery. Transformers work by way of electromagnetic induction.
Electromagnetic induction is the result of a changing magnetic field which induces an electromotive force (emf) or voltage in a conductor (wire). In the case of a step-down transformer, AC electric current flowing through a primary coil of wire creates a changing magnetic field. A secondary coiled conductor (with a smaller number of turns) that is closely exposed to the changing magnetic field of the primary coil will be charged with a electric potential (voltage) which is lower than the voltage applied to the primary coil.
The voltage decrease in the secondary coil (in a perfect world) is directly proportional to the number of turns of the primary coil compared to the number of turns in the secondary coil.
e.g. 1000 turns on the primary and 100 turns on the secondary would produce 1/10th of the voltage in the secondary coil compared to the voltage that has been applied to the primary coil.