The following is an Evangelion Episodes
25 and 26 version of Instrumentality
which doesn't take into account anything revealed in the movies, parts are quoted directly from the AD Vision dubbed
The Complementation of Man Via the Instrumentality Begins
The End of the World
Once again, Shinji Ikari...
Shinji Ikari: What... What feeling is this? I think I've experienced this sensation before... It's as though my physical form was disappearing. It feels... agreeable... As though my existence is spreading itself out to cover the entirety of existence...
This is the beginning of Instrumentality.
That which the people had lost...
...the minds which had disappeared...
...is rushing in to fill the void in the Mind.
The complementation, Instrumentality for the Mind and Soul, begins.
Instrumentality, which will return all things to nothingness...
Gendou Ikari: Incorrect. It is not a return to nothingness. It is merely a return to the state of beginning. It is no less than a return to the primal womb that we lost so long ago. Souls and minds will become one, attaining eternal balance. The final goal is nothing more than that.
Misato Katsuragi: And that's Instrumentality.
Ritsuko Akagi: Yes. There is an emptiness at the very core of our souls. A fundamental incompleteness that has haunted all beings since the very first thought.
This lack causes a void within the mind.
This void is the cause of anxiety and fear.
Ritsuko Akagi: On a primal level, Man has always been aware of the darkness that resides at the core of his mind. We have sought to escape from this void and the fear it causes, as all Man's accomplishments were made in the hope of filling it.
What is above is a brief description of Instrumentality, you are not expected to understand it at first... It takes a great deal of thinking and questioning before you even begin to near that state. That is part of the brilliance of Neon Genesis Evangelion, and in the end we can all learn something from it.