A transformer is an electrical device that has two inductively linked conductors. When a voltage is applied across one of the conductors there is a voltage induced in the other conductor.
Step up and step down transformers have two coils, a primary coil across which a voltage is applied and a secondary coil across which a voltage is induced. By varying the number of turns in the two coils the ratio of the number of turns in the secondary coil to the number of turns in the primary coil is altered. This ratio is equal to the voltage induced in the secondary coil to the voltage applied across the primary coil.
If a transformer has 110 turns in the primary coil and 330 turns in the secondary coil it is a step up transformer. The voltage induced across the secondary coil is 330/110 = 3 times the voltage applied to the primary coil.