EMF stands for electromotive force.
We all know about electrolytic cells in which electricity is produced by the difference in potentials of anode and cathode . Thus their a potential difference is created.This maximum potential difference is called Emf. But actually most of the cells are not able to provide potential difference equal to the EMF. This is due to the resistance present in the electrolyte and hence a little of the potential difference is lost so actually EMF is never achieved but in an ideal case it EMF is the maximum potential difference.