To release an electron from a metal plate required a energy above a certain value. That energy was called the 'work function energy'. It was the energy required to do work against the structure of the metal that the electron was part of. An electron on the surface of the metal (the least tightly held one in the structure) was bound to the metal by that energy and to release it you had to input that amount of energy. Each metal has a different structure and so was found to have a different 'work function'. Those that held on to their electrons lightly had low work functions those that held on tightly a high one!The electrons could not absorb more than one photon to escape from the surface, they could not therefore absorb one quanta and then another to make up the required amount - it was as if they could only embrace one quantum at a time. If the quantum absorbed was not of sufficient energy the electron could not break free. So 'escape energy' could only be transferred by a photon of energy equal or greater than that minimum threshold energy (i.e. the wavelength of the light had to be a sufficiently short). Each photon of blue light released an electron. But all red photons were too weak. The result is no matter how much red light was shown on the metal plate, there was no current.