Light is incident on the cathode. For a lower intensity light, the current still drops to 0 at the same place. This is called the stopping potential. A different frequency of light will have a different stopping potential. Each electron is assumed to only be able to absorb a quantum of energy.
Photon energy is a function of the frequency.
Meaning that the yellow 'photon' for a particular metal may not have enough energy to give up. Yellow Light if it is enough to cause the effect, produces 'slow' electrons which are easily pushed back by the Variable Pot. The electron takes the whole photon and keeps what it doesn't need as kinetic energy. Photons of the same 'colour' will all produce electrons with the same speed, meaning that they will stop at the same point. Photons of different 'colour' will produce electrons with different speeds giving different stopping potential.