It is a normal fact that the increase in load to a decrease in the supply voltage. Hence voltage supplied by the transformer to the load must be maintained within the prescribed limits. This can be done by changing the transformer turns ratio.
The taps are leads or connections provided at various points on the winding. The turns ratio differs from one tap to another, and hence different voltages can be obtained at each tap.
Tappings are provided on a transformer winding for selecting/cutting out a certain number of turns on the transformer winding thus obtaining a variable turns ratio. This is done in order to maintain the output voltage within desirable limits because the equipment works satisfactorily at a somewhat fixed voltage and voltage variation is a common phenomenon in the power system.
Tapping of a transformer means the connection point along the transformer winding to select the suitable number of terms. By Tapping, you can get different turns ratio and be able to control output & voltage regulation.