Light is made up of particles called photons.
These photons have some kinetic energy, just like any moving object. But for most objects, kinetic energy is related to how fast the object is going. Since all photons move at the speed of light, something else must determine the amount of energy carried by each photon. It turns out that in certain ways photons tend to act like waves, and that each photon is 'waving' at a certain frequency. The energy of the photon is directly related to this frequency.