A transformer is a device that changes the voltage of electricity.
Transformers are fairly simple devices that operate under the principle of magnetic induction. A transformer consists of primary coils, which are located on the side of the transformer nearest the generator; secondary coils, which are located on the side of the transformer located nearest the customer; and a hollow iron core in the center. The electric current in the primary side induces a related current on the secondary side based on the principle of electromagnetic induction. The ratio of coils from primary to secondary is what determines the voltage output of the transformer. A transformer with more coils on the secondary side is called a step-up transformer because it increases the voltage. Alternatively, a transformer with more coils on the primary side is called a step-down transformer as it reduces the voltage.
Watch this short video to learn more about transformers: