The higher knowledge relates to the Immutable Brahman, as noted in verses 6 and 7 of the 1st section of chapter 1 of the Mundaka Upanishad, as translated by Sri Aurobindo:
“That the invisible, that the unseizable, without connections, without hue, without eye or ear, that which is without hands or feet, eternal, pervading, which is in all things and impalpable, that which is Imperishable, that which is the womb of creatures sages behold everywhere. As the spider puts out and gathers in, as herbs spring up upon the earth, as hair of head and body grow from a living man, so here all is born from the Immutable.”
The Upanishads use a lot of “negative” attributes to describe the Immutable Brahman, in order to avoid the mind fixing on a specific characteristic and thereby trying to limit the Brahman by that characteristic. The positive characteristics that express the unlimited nature of the Brahman are immediately associated. The Brahman transcends all created objects and forces, it acts as their container, their womb, and their content.
The images of the spider, the herbs and the hair all illustrate how the manifested universe is based on and is created by, and from, the Immutable Brahman. They are not separate or independent, but actually embody the Brahman in form, without thereby limiting the Brahman in its all-pervasive, infinite and Immutable form.
The Upanishads make it clear that we cannot grasp the Immutable with our minds or our words. They also make it clear that there is no duality: “one without a second” and that the names, forms, and energies that go into the world we live in are manifestations of that sole Existent: “all this is the Brahman.”
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210