Planck's constant is a fundamental physical constant that relates the energy of a photon to its frequency. It is denoted by the symbol h and has a value of approximately 6.626 \times 10^-34 joule-seconds. This constant is crucial in the field of quantum mechanics, as it helps explain the behavior of particles at very small scales. In the context of quantum theory, Planck's constant signifies the smallest action or energy change that can occur. It plays a key role in the photoelectric effect, which was explained by Albert Einstein, demonstrating that light can behave both as a wave and as a particle.