Think of this as passing an exam. The threshold to pass is 50% (or in the example, 3 x 10^-19 Joules is the threshold energy). You can submit millions of 49% exams, but you still won’t pass. However, just one exam at 50% will pass. Millions of red light photons can be directed at this material, but it won’t “pass” and emit a photoelectron. On the other hand, one blue photon alone can emit one photoelectron because it has enough energy.