The Sefirot were an innovation in that they are visible lights, whereas Eyn Sof is not visible.
The Sefirot are lights that were permitted to be seen, which is not so in the case of the simple light of Eyn Sof, blessed be He.
The subject of all the investigations of the Kabbalah is the Sefirot. Therefore this is what must be explained first.
The proposition consists of two parts. Part 1: The Sefirot are lights…This tells us what the Sefirot are. Part 2: …which is not so in the case of the simple light. This tells us in what way the Sefirot are different in their nature from Eyn Sof. For in order to understand the Sefirot, it is necessary to understand in what way they were an innovation introduced for the sake of the creation: this is explained in Part 1. Initially, however, there was only Eyn Sof, blessed be He. If so, in order to know the Sefirot, it is necessary to know in what way they differ from their original state, and from Eyn Sof.