Ein Sof means, word for word, "without end." It is not a name of G-d in the sense of the Torah, which knows several. It is rather a refusal to name: the way the kabbalists point to G-d as He is in Himself, before any relation with a world, before any attribute. The Pardes Rimonim reminds us that even "first" is too small a word, since it implies a beginning; Ein Sof is first without having a start.
Etz Chaim, where Chaim Vital recorded the teaching of the Ari, opens its great narrative this way: before the emanated were emanated and the created were created, a simple supernal light filled all of reality. No empty place, no free air, neither head nor end, but a single simple light, equal at every point. This is what is called "the light of Ein Sof." Keep the two words: simple, that is, without parts or degrees; equal, that is, without above or below, without right or left.
The most famous passage on this subject is the prayer of Elijah that opens the Tikkunei Zohar, "Patach Eliyahu." The prophet addresses the Master of the worlds: "You are One, but not in number." Cordovero explains the phrase: the one that we count always has a possible second beside it; the One Elijah speaks of has no second, not because He wins a comparison, but because no comparison applies. Then Elijah adds: "No thought grasps You at all." That is the limit set from the start: Ein Sof cannot be thought, cannot be described, and all the Kabbalah that follows never speaks of Him directly, only of the way He lets Himself be known.
Why speak of Him at all, then? Because the question running through this whole tradition is precisely this one: how can a finite world, with its limits, its forms and its faults, come from an Infinite without limit? The five entries that follow are Safed's answer, step by step. The Pardes insists on a point you must keep in mind: Ein Sof is not Keter, the first Sefirah. To confuse the two, says Cordovero, is "to cut and to separate." The Sefirot are vessels and instruments in which He clothes Himself to act and make Himself known, as the soul clothes itself in the body; Ein Sof remains beyond them all.
Keep this for what follows: whenever you read "light," "vessel," "line," it will be about the way the Infinite limits Himself willingly so that someone other than Him can exist. Ein Sof does not change. It is our capacity to receive that is put in place, degree after degree.
