I really don't get exactly what the Human Instrumentality Project was. Like...was it a form of therapy? Someone explain, please.

While I'd like to be... not an ass, I'd say the "..." attempt sort of bars the inquiry from gratification. But I'm sure there are those who are just so damn eager to bite the hook, it will probably get an answer in the form of a friendly brawl anyway.

HIP was a bunch of Neo Nazis' nefarious attempt to convert the entire world's penguin population to scientology. However, things went horribly awry, and they all became Mormans instead, after which they relocated to Salt Lake City. There, the penguins' religious rites unexpectedly opened the gate to another universe, from which a hot iron 1/4 the size of the moon emerged and, before any measures could be taken, flattened the entire North American continent, killing all life upon it.

Maybe I shouldn't browse forums when I'm so tired.

A-a-anyway, answering the original question...
Here's the basic idea:

The Human Instrumentality Project, intends to start an artificial evolution of mankind. SEELE is the main driving force behind this project, for reasons unknown, but they mention that humanity must evolve or it will die, thus the need for a forced evolution.
This artificial evolution strives to merge all human souls into one by disposing the individuals of their AT-Fields, which causes their bodies to revert to LCL. When everyone comes to this state, they will no longer feel the pain or loneliness that would typically percipitate from interaction between humans; it is camparable to death.

The first paragraph was writen by anoher person, but I wrote the last one. You can find the whole thing at Wikipedia.

Here's the official statement from the Red Cross Book:

Red Cross Book wrote:A plan to induce artificial evolution for human, who had reached an impasse in becoming a colony of worthless, to become a total single-form organism. It was under the direction of SEELE, and the secret agency NERV was the implementation organization. However, the expectations of SEELE seems to be different from the objective of NERV (which means the objective of Gendou and Fuyutsuki). EVA was in fact not planned as a weapon. It was built with the aim to realise this plan. More specifically, it seems that the plan is to artificially start the Third Impact, thus erasing all human, and after human shed their physical form, they will then evolve into a new stage.

Hopefully, I've answered most of the question, but anyone can add to it to help you understand. :)

MAGI wrote:This artificial evolution strives to merge all human souls into one by disposing the individuals of their AT-Fields, which causes their bodies to revert to LCL.

Amazingly enough, the soul-ATF-body link isn't as direct as everyone likes to think.

didnt kaworu say: "the at field is the light of the soul" or something? hmm..
and also, i must clarify that fuyutski didnt necessarily want to follow the HIP, but gendo was the leader, and he kind of followed him along...
although one can argue that fuyutski wanted to be with yui....
and also, seele might have been using the "saving the human race" as an excuse, as they just didnt want to truly die...

sadsadshinji wrote:didnt kaworu say: "the at field is the light of the soul" or something?

YEah i thought i heard him say something of the sort also.

sadsadshinji wrote:didnt kaworu say: "the at field is the light of the soul" or something? hmm..

I wasn't debating a connection between the ATF and soul. I was debating the idea that the ATF holds one's body together.

Yeah, I'm not sure. The Red Cross Book says "With this A.T.Field, human is able to keep separate from each other, and to stay in seperate physical forms." So, that's our best bet, for now.
So, maybe think of the AT-Field as something that holds our bodies together, but, if one's is powerful enough, can extend outside of the actual bodies to form an invisible barrier. If another has an anti-at-field, then it can make that disappear. If there is another anti-at-field strong enough, it can make the actual body disappear. So by having the light of people's souls disappear, they can't hurt the others, and they can't repel the other souls from coming together. The light is a metaphor, anyway.
Makes sense.

MAGI wrote:Yeah, I'm not sure. The Red Cross Book says "With this A.T.Field, human is able to keep separate from each other, and to stay in seperate physical forms." So, that's our best bet, for now.

The nice thing is, the show directly contradicts idea this (edit: speak not I can) by having people running around, bodies intact, after Anti A.T. Fields manifest -- during both Impacts. By the same token, even after Rei brings ATFs back, most of humanity remains a giant puddle of Tang. Direct soul-ATF-body connection, indeed. But if anyone wants to see anything else about this discrepancy, I'll just post links to the relevant threads.

Reichu wrote:There, the penguins' religious rites unexpectedly opened the gate to another universe, from which a hot iron 1/4 the size of the moon emerged and, before any measures could be taken, flattened the entire North American continent, killing all life upon it.

you are wrong Raharu stoped it.