This is analogous to the rays of the sun that shine from the sun. The rays are the ability, from within the sun, to shine upon the earth. And the sun itself refers to the actual solar globe.
The light that shines forth from the sun is called “Light,” as the verse states (Bereishis 1:5), “And Hashem called the light ‘day.’” Day is called ‘light’ due to the light rays that shine forth during the daytime.
However, the sun itself is not called ‘light,’ but rather, “the luminary” or the source of the light. As the verse states regarding the sun and the moon (Bereishis 1:16) “The two great luminaries, etc.”
This is the idea of Hashem’s Infinite Light: The “light” refers to the revelation from which light and life exists and extends forth.
And, accordingly, the Infinite One Himself is the “luminary,” the source of the light, from which the light extends.
Now, the Tzimtzum-Withdrawal was only in the Infinite Light, meaning, that Light retracted to become included within the Luminary.
The Tzemach Tzedek (Likutei Torah, Vayikra 52d) explains this:
When a person is by himself, he generally communicates to himself through thought. However, sometimes he may speak over to himself his thoughts, to help concretize a concept. (In fact, when we learn Torah, even by ourselves, we need to verbally recite our learning in order to internalize it and help us remember it etc.)
Similarly, as it were, before Hashem created the world, He had the “potential” for Infinite Light in Himself. This is like a person’s thoughts, which are the potential for revelation in speech. In addition, Hashem had “self-speech,” a revelation of His Infinite Light, of His infinite power to create and give life. The effect of the Tzimtzum was to “retract” this self-revelation of His Infinite Light, to revert to the “potential stage,” like a person who stops speaking and retracts his “self-revelation” back into his thoughts.
