If God's light were to flow into the world without restriction, this world could never exist. Everything finite would dissolve back into the Infinite like a candle flame in the sun. Rabbi Schneur Zalman of Liadi uses this insight to explain the most foundational concept in Kabbalah: tzimtzum (צמצום), divine contraction.
Consider the name Ein Sof (אין סוף) itself. It means "without end." No limit. No boundary. No finitude of any kind. If the worlds had descended from this light in a straightforward way, through gradual cause and effect, nothing finite could ever have come into being. Even the World to Come, even the highest souls and the loftiest angels, exist within boundaries. They can only absorb a limited measure of divine light before they would be nullified out of existence entirely.
So God contracted His light. Not in the sense of physically withdrawing, but through what the Tanya describes as "occultation and concealment." Layer upon layer of cosmic screens that filter the infinite radiance down to an "exceedingly contracted illumination" that creatures can survive.
How contracted? The Tanya reaches for mathematics. The number one has a relationship to one million: it is one-millionth of it. But in relation to infinity, even a billion or a trillion has no relationship at all. It is "veritably accounted as nothing." The light that sustains our world is that kind of nothing compared to the uncontracted light of the Ein Sof.
And yet God is fully present here. The contraction does not diminish God. It conceals God, the way a thick curtain does not reduce the sun, only blocks its light from entering the room. Pull the curtain aside, even slightly, through Torah and mitzvot, and the full infinite light is right there, waiting.