Comet brightness is measured by the same magnitude scale as stars and other astronomical objects. The smaller the number the brighter the object. The dimmest star you can see in a totally dark sky is about magnitude 6, Sirius, the brightest star is magnitude -1. A comet of a given brightness is usually harder to see than a star of the same magnitude because the comet is diffuse - its light is spread out over a small area, while a star concentrates all its light in a single point. Comets become fairly easy to see with the naked eye at around magnitude 4 or 3.
