Brightness is the relative intensity as perceived by the average human eye. The frequency of a light wave is related to its color. Color is such a complex topic that it has its own section in this book. Monochromatic light is described by only one frequency. Laser light is effectively monochromatic.
Polychromatic light is described by many different frequencies.
Nearly every light source is polychromatic. Although frequently associated with light, the term can be applied to any wave phenomena. A continuous spectrum is one in which every frequency is present within some range. Blackbody radiators emit a continuous spectrum. A discrete spectrum is one in which only a well defined set of isolated frequencies are present.