The Sefirot were an innovation in that they are visible lights, whereas Eyn Sof is not visible.
The Sefirot are visible lights of His Will — what He has chosen to make seeable to creatures, in contrast to His own unseeable simple light.
The foundational quartet (Op. 1–4) has set the axiom, the goodness, the purpose, and the plan. The book now turns from what we may say of God to what we may say of the structure He revealed. The first thing in that structure has a name: the Sefirot. Op. 5 introduces them by category. They are visible lights — not God Himself, not separate beings, but what He has made seeable.
The word Sefirah (singular; Sefirot plural) comes from a root meaning to count or to number. The very name suggests something measured — finite, articulable, subject to enumeration. This sets up the contrast Op. 5 draws. Eyn Sof — the Limitless, a name for God Himself in His infinitude — is unseeable. There is nothing measured about Eyn Sof; there is no boundary at which He stops, no shape He fills, no number He matches. His simple light is not light in the sense of something that can be perceived. It is what makes perception of anything possible at all, but it is itself not a perceived thing.
The Sefirot are different. They are lights in a sense closer to ordinary use: they have measure, they have form (though as we will see, the form is not intrinsic), they can be perceived by prophets and contemplated by students. The crucial qualification is in the word permitted. A Sefirah is not a new substance brought into being. It is the same Godliness, made visible to creatures. The change is in revelation, not in nature.
Op. 5 makes this point with a careful pair of contrasts. The Sefirot are an innovation — something new in the order of revelation, not new in the order of being. They are visible — which means He has chosen to make them so. They are lights — which carries the everyday connotation of something that illuminates, but stretched into a cosmic register where what they illuminate is the cosmic order itself. And they are lights of His Will — which holds them firmly within the restriction Op. 1 set: we are speaking of His Will, never of His Essence.
The payoff of this carefully-set structure arrives in the next chapter (Op. 6), where the content of each Sefirah is specified as one of His attributes. For the moment, hold the category. The Sefirot are visible lights of His Will, in contrast to His own invisible simple light. They are how He shows Himself; they are the seeable face of an unseeable Will; they are what makes the rest of the cosmic discussion possible.
If you find yourself wondering how exactly a Sefirah relates to Eyn Sof — not separate, not identical, somehow intermediate — that question is exactly the one the next chapter will sharpen. For now, hold the picture: visible lights of an invisible Will.
Op. 6 will tell you what each Sefirah is in content (an attribute of His Will). Op. 17 will distinguish a Sefirah (one of the ten powers, the foundation) from a Partzuf (the same power seen in operational detail) — a distinction that runs through the entire second half of the book. And Op. 25 will explain cosmogonically how the Sefirot become visible: the Tzimtzum is what makes the Sefirot apprehensible by removing the limitlessness that would otherwise have made them invisible.