I can't tell you what it was about, but I'll share with you what I took from it. I love to write, and I am hopelessly idealistic and tend to romanticize everything beyond any rational recognition so take this all with a grain of salt.
The creative type (Javier Bardem's character,
Him) needs inspiration (Jennifer Lawrence's Character,
Mother) to create something wonderful. Anything.
The end.
No, I'm just kidding.
Sort of.
The rest is all just visual symbolism showing the dynamic between such a poet and his muse. He loves her, but mostly only for the inspiration and stability she provides him while he tortures himself writing, pining for the adoration of his fans. Everything appears to me to be the struggle of that dynamic. She loves him. He loves her, but really what he sees of her is just some imagined morbid idealism that he's conjured up to project onto her for the sake of being inspired to create (the crystal. it's not
her but what he finds inspiring
within her that he cares for, and that's interchangeable as we see later in the movie). I think that's why the character is a different actress in the end. I mean to help reinforce that "she" is not as important to him as what she brings to the table as inspiration. So, he resets and moves on to another woman to place his projections onto for his next creation. All of the conflict, the heart, the war zone, the fire, the cult worship of celebrity, etc., I imagine is representative of the struggle one might face when truly loving someone caught up in that head space or career life (directors, actors, writers, musicians).
"I am your muse. Stay with me and love me, and I will keep you grounded and stable and I will inspire you to create wonderful things... for as long as you love me too. And I will believe, naively, that you do love me because I love you so very much that I will convince myself so. What? You don't need me now? My heart is breaking. I know that is OK for you though. You will just move on to another and live off of her new adoration like an emotionally unstable and insecure poetic parasite for it is not love that you desire. It is, instead, inspiration and fan adoration."
I personally feel this movie was just an attempt to translate all of this confusion, frustration, love, inspiration, and devastating heart break into a model to try to make some sense of it all through and abstract visual narrative.
This is also a two-way street though. I mean, it sounds so cruel and heartless, but the fact that a writer can step outside of his or herself to empathize with those partners that may have been manipulated and taken advantage of for a selfish creative nudge, in and of itself shows a level of self awareness and possible guilt that may not have been so obvious at the time through experience. Later though, this could have been discovered and investigated through self-reflection.
The story is about the poet's world, told through the perspective of his muse. The writer would have to be aware and considerate of this dynamic to even begin to translate this to story. I imagine there was a lot of personal and emotional closure in creating this movie.
Eh. That's what I got from it all. But who knows

@
gunsnfish
EDIT
TLDR: He burns a relationship by taking everything he can of her for inspiration until she literally burns out and leaves. He then supplants that inspiration onto another to do the same because he just needs stability and inspiration for his work. Rinse and repeat forever or until the guy realizes what he's doing and cares enough to bother stopping.