This is a property of a wave, and it is the number of wave crests that pass a given point per second. Frequency is is measured in units of inverse time (e.g., ``cycles per second''). A cycle per second is the unit of frequency and it is known as a ``Hertz.'' Since light moves at the constant speed of light, the frequency of a light wave is related to the wavelength: the frequency is given by the number of wavelengths that go by per second at the speed of light, hence frequency is wavelength (distance) divided by speed (c). The higher the frequency of light the greater its energy.