3 core aspects of existence are:
1-substance
2-patterns
3-motion

motion changes the patterns which are made of the substance

this is existence

nothing is separate from it

all 3 must be eternal

human identity (separate self with free will) is a kind of illusion that "sits ontop" of this deeper omnipresent reality. but can be a core part of the illusion of human life, and so necessary for the game to play out.

god is playing peek-a-boo with himself. he hides as a human then sees his real self

It can be helpful to bear in mind, when discussing this distinction that we are moving within the field of existentialism - although Sartre never wanted to be known as an existentialist. What Sartre is attempting here is a distinction between "thing" (comparable to Kant's noumenon) and subject.
Being-in-Itself (the "en-soi") refers to all non-human entities / objects / material things. This category of being represents real / actual existence: these "things" are what they are. In other words, the are not what they can become. Their existence coincide / is identical with what they are at any given moment. Although these existents represent actual (full) existence they are not aware of this being the case as they lack consciousness of anything, also of their being in existence. In contrast to this category of existence Being-for-Itself (the "pour-soi") is aware / conscious of their being in-the-world. These existents (subjects) are not what they are, because they are forever what the can become. Not being fully existing means that in the awareness of the full existence of any "thing", there is at the same time also the awareness of not being that thing. For this consciousness of not being this or that thing Sartre uses the term "neantisation": in my being of awareness of a thing I am at the same time also "neantisitating" that thing by affirming my being not that (thing).
In making a deeper analysis of this Sartrean distinction you will definitely find that it becomes problematical - so much so that its validity can be questioned. Hope my attempt to give this short overview of the distinction gives you a better idea of what it entails.

Daniel, thanks for your comments.

In line with your concluding paragraph, how can being-for-Itself (you and I) be so certain that being-in-itself (the noumenon) lacks consciousness?

Doesn't Sartre claim that being-for-itself (consciousness) originates through, what he characterizes as, a "decompression" of being-in-itself?

How can a being that is defined as "infinitely dense" undergo a perpetual spontaneous self-decompression? Are we to assume that it "decides" to decompress itself, or, instead, does it undergo decompression because it is perpetually nihilated by a simultaneous being-for-itself?

But I thought that a nihilating being-for-itself (consciousness) could only originate from being-in-itself?

Very confusing!!!!!!!

Charles, I agree. Sartre can be very confusing and even self-contradictory. Sorry than I am not able to give explicit answers to your questions. However, there's one thing about Sartre I always remind myself of when struggling to understand him: everything he says about consciousness, being, nothingness, the subject, objects and others always presupposes (sometimes only implicitly) "being-for-itself". There's never a chance for him getting away from this "fact"!

What does it mean when Sartre says that Being-in-Itself "is what it is," whereas Being-for-Itself "is not what it is and is what it is not"? — charles ferraro

The chair is a chair. When I look at the chair, it fills my consciousness. 'I' am that chair, and yet I'm not. What am I besides that chair? I am the witness of various objects and yet none of them. But I am nothing other than those objects. So I am all objects and yet none of them. Even the 'I' is an object for the witness. So is the concept of the 'for itself.' The idea of a subject, of consciousness, is actually troubled by this thinking. Being and consciousness are one.

And yet we know that it's somehow a picture show in an individual skull.

I've never seen this tension satisfactorily resolved. We have two starting points and interesting work from each point.

I know next to nothing about Satre’s ideas. I can say that I do tend to carry around my ‘world view’ as ‘me looking at myself’. What I ‘see’ is only ever ‘myself’. I don’t mean this in a solipsistic way, but more as ‘how I see’ is ‘me’, and the ‘me’ is only ever a temporally directedness rather than some explicit ‘point’ with inward/outward and/or future/past facing.

I love the idea of using the Greek Titans Prometheus and Epimetheus (forethought and hindsight) to express our conscious regard - sense of ‘existing’.

Nausea is a great little novel. There are lots of great passages in Being and Nothingness. And his Baudelaire is fascinating. To me Sartre is fascinating personality. And sometimes he just nails down the eeriness of the human experience in just the right words.

I like the 'how I see' framed as the self. The self is like the distortion of the world, its bentness. It's like those old windows one sometimes sees. The glass isn't perfect.

I also like forethought and hindsight as themes. Time is central. As others have said, man 'is' time. ANd perhaps it's all in your experience of this very sentence, as you anticipate the ending that will inform your experience of the beginning. Bennington's book on Derrida brought this kind of 'reading time' to my attention. 'I think therefore I am,' is one more sentence caught up in 'primordial' time.

My thought is me: that is why I cannot stop thinking. I exist because I think I cannot keep from thinking. — Sartre

And this one is funny.

If I became a philosopher, if I have so keenly sought this fame for which I'm still waiting, it's all been to seduce women basically. — Sartre