Why this movie is so weird, even for sci-fi fans
An attempt at an explanation:
Most sci fi stories still follow the model of classic allegory with clear protagonist and antagonist. This is blurry in Solaris as the film is actually dealing with a collision between two fundamentally different philosophies of existence. A very brief outline of these philosophies follow. Carry on if you dare:
Most science fiction, perhaps more than most genres, is firmly grounded in Enlightenment principles: that there is a reality "out there," independent of human presence. There is matter and energy and that those things follow predictable, observable and rational laws. In such a tradition, there are natural, mechanistic laws in which humans are merely a part and inseparable from. It was probably Isaak Newton and his establishing physical laws that struck the greatest blow on how society viewed nature and our place in it in this regard.
But in Germany, starting in the late 1700's there was a loose philosophical and aesthetic backlash to the rigid natural laws on Enlightenment thinkers. Although these Philosophers (such as Fichte) are much less well known, their take on reality has been deeply influential on how we view nature and our role in it ever since. These are what are now identified as the Romantic thinkers.
The Romantics resented and rejected the constraints on free will and the deterministic, mechanistic interpretation of human existence based on rational, natural law. If man was merely a biological machine governed by social, biological and physical law, and operating within a larger mechanistic universe governed by physical and chemical laws, then Free Will is a farce. They rejected this.
Their idea was that the fundamental, meaningful "stuff" of existence was spirit (or will, or consciousness, or purpose), both individual will and a collective will of the universe. The key point is that this spirit, which is what defines humanity and the greater reality, is not constrained by any such rational rules or laws. Man wills and acts to create his own nature, and it is those acts of will that create art, culture, societies, etc., basically, everything that is important and meaningful.
For the Enlightenment thinker, man and his society is the mere consequence of natural laws of the universe. For the Romantic thinker, man and his culture and society is the product of his individual and collective will. In the former view, reality is dictated to us, in the latter view we makes our own reality.
You may notice that the Romantics were the "proto-psychologists" and are the precursor to Freud (through Nietzche) and his idea that man is largely influenced by an irrational subconscious (replacing the terms of spirit or will with subconscious or "Id").
Most modern people carry a philosophical duality in attitudes that incorporate both incompatible ideas, that there is a physical universe "out there" that is governed by rational laws, but also that there is a human will that is not rational and is "free" to create and imagine.
In the movie Solaris, the two philosophies are allowed to collide by positing that the Solaris entity can and does physically manifest that inner spirit, the subconscious world of a person. Proximity to Solaris results in the irrational subconscious projections of a human mind being made into an organic being. Solaris is thus, some kind of gateway linking the "pure mind" to the classical physical world.
The initial "earth world" in the story is strictly rational. It follows all of the scientific rules and laws that almost all science fiction stories assume, and then crashes that reality into a world that is a Romantic's wet dream, where the rational rules of nature are not helpful in orienting us. Mental and emotional mayhem result.
Solaris is a world grounded in spirit, and that spirit world has its own forms of creation, procreation and existence. While there is considerable violence perpetrated between the "Earth people" and the "Solaris Creations," the "action" in the movie Solaris is largely psychological and philosophical. We watch as the two systems strive for equilibrium. Is a stable state reached at the end of the story and which world wins? It is meant to be thought provoking, to stimulate discussion, and these kinds of movies are not for everyone.