Answer
It is a large encased group of metal plates about the size of a small house with 2 sets of insulated copper wires wrapped around the plates -an input and output - to convert a very high supply line voltage - maybe 44 thousand volts to a lower voltage of 2200 volts that runs down the power line poles in your neighborhood. Look at your street's power poles and you will see a mini substation that converts he 2200 volt to 220 volts and may supply several homes.
Actually a substation transformer for a distribution substation will reduce the incoming voltage from 115,000 volts or 69,000 volts to a lower voltage closer to 7200 volts that runs down the power poles in your neighborhood. At your house there will be a small transformer that reduces the voltage from 7200 volts to 240 volts to meet the needs in your home.
Actually both are wrong !!