What is realization, and what happens when you reach there as per Upanishads?

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As per the traditional belief, whatever truth one finds in the Upanishads is an attempt by the Upanishadic sages to explain their experience, that they had when they were in deep state of meditation. That is the realization they arrived at. It is a different matter whether they could describe their experience exactly as they went through it. But anyone who goes on their path, would surely experience the very same truth and it needs no further validation.

What is this realization as per the Upanishads?