What is an A.T. Field?
There are things we do not wish to admit to ourselves.
There are lies that we tell ourselves to keep our sanity.
There is knowledge so horrifying that our own subconscious suppresses it to keep ourselves from knowing it.
This is the Absolute Terror Field - the border of the mind that we dare not cross.
In the series Evangelion, the physical manifestation of the A.T. Field made the Angels undefeatable by conventional means. Only by breaking apart their own A.T. Fields and revealing to themselves the truth that lies within can the pilot of an Eva unit sunder an opposing A.T. Field.
And you wonder why Shinji is so terrified of that cockpit...