Vertical power delivery involves putting the voltage regulators on the opposite side of the circuit board of the chip, so that high-current power supplies travel a minimum distance. This reduces both resistance and inductance of the paths that power takes to the transistors, which means significantly less loss. That, in turn, means less heat related to those losses and less infrastructure to prevent those losses (fewer bypass capacitors, etc), so more of the thermal and area budgets can go to compute.