Planck's Constant, denoted as h, is a fundamental quantity in quantum mechanics that relates the energy of a photon to its frequency. It plays a crucial role in the theory of quantization, which describes how energy is emitted or absorbed in discrete packets called quanta. This constant is approximately equal to 6.626 x 10^-34 Joule seconds. It is essential for understanding phenomena such as the photoelectric effect, where light can eject electrons from a material. The significance of h extends to various fields, influencing our comprehension of atomic and subatomic processes.