The stopping voltage is the voltage applied across the photoelectric plates to measure the maximum KE of ejected electrons. This is as the electric field will apply a force against the velocity of the electrons and thus slow them down before they reach the other side.
The issue I have with this explanation is that once the electrons have been slowed down mid air, they have to go back to the plate they came from and they terminate with the same velocity they left with. Just like how this Kinetic energy creates current on the RHS plate, shouldn't returning electrons thus create a negative current? I suppose it might have something to do with the fact that other electrons are being shot out at the same rate, forming some kind of equilibrium with the old electrons filling in the holes?