Each Sefirah is one of the attributes of Eyn Sof, blessed be He, with which He created the worlds and with which He governs them. Since He wanted that they should be known, He made each attribute appear as an individual light, through seeing which one may understand that attribute. When one sees the movements of that light, one understands the influence of the corresponding attribute on the government of the worlds at that time.
Having learned that the Sefirot are lights or radiations, we must now understand what these radiations are.