After lamination of the core of a transformer eddy current reduces. Because transformers are basically two coils of wire wrapped around a core of iron. They work by induction. Induction works when current flows in one conductor and the magnetic filed forms around that conductor sweeps the other conductor and induces the voltage. If we didn't laminate core, the core would provide a place for the magnetic lines to produce current, and that current in the core would heat and the core really fast and waste energy. So by laminating the cores, we break up the current paths within core and limited eddy current.