As seen earlier, the focus of all the Upanishads is the realization of Brahman. But this concept of Brahman is difficult to understand. Two descriptions of this are given below. According to the Kena Upanishad, it is through Brahman that everything is known. Yet Brahman is neither the known or the unknown.
The Katha Upanishad says: ‘Brahman, the immortal, contains all worlds in it, and no one goes beyond it.’
Some Upanishads state Brahman has two forms, both mortal and immortal. The mortal form must refer to the gods, who, though a part of Brahman, are not eternal.
Brahman, in its true sense, has never been created and can never be destroyed.
Some of the later Upanishads focus on gods, others on rituals. There are yoga Upanishads and sannyasi Upanishads. But the aim of all is the same, to transcend the world and realize Brahman. For this one must first understand the bliss of true realization, for only then will one focus on it. And only when the mind is fully focused on Brahman and on nothing else, will such realization be possible.
For more on the Upanishads, read The 108 Upanishads depicted above.