Transformer turns refers to the ratio of the number of turns in the primary versus the secondary of the transformer. This is discussed in other articles in this blog listed below. Many transformers are designed with tap switches which allow one to change the effective number of turns in the primary side of the transformer. This changes the turns ratio either up or down depending upon the direction of the change (up or down). In turn the voltage increases or decreases which will increase or decrease the amperage in the secondary. The result is that the tap change has increased or decreased the total power available in the secondary depending upon the tap chosen.
Other articles on Turn Ratio in this Blog:
Reference: RWMA - Resistance welding Manual 4th Edition