Symbolism, Imagery, Allegory
The Visitors are constructs plucked from the scientists' fantasies and dreams and memories. So guess what? They symbolize fantasies and dreams and memories.
Easy enough, huh? Who said this Solaris stuff was hard? Oh wait, we kind of did—and this is definitely true when it comes to the black woman visitor.
The Black Woman
The first visitor Kelvin sees on the station is a black woman, or, as Kelvin says, "A giant Negress was coming silently towards me with a smooth, rolling gait" (3.9). He goes on to say:
I caught a gleam from the whites of her eyes and heard the soft slapping of her bare feet. She was wearing nothing but a yellow skirt of plaited straw; her enormous breasts swung freely and her black arms were as thick as thighs. (3.9)
He adds that her grass skirt resembled "one of those steatopygous statues in anthropological museums." FYI: The word steatopygous means having fat or prominent buttocks.
Kelvin's initial alien encounter, then, is with a stereotypical, enlarged, supposedly primitive black African woman, dressed in sexualized native garb. Kelvin refers to her with the racist word "Negress" and focuses on her rear end. This woman, then, is not only alien because she is alien—she is alien because she is black, and because she is a woman, and because she is a black woman. She defines difference and otherness, in short, because of racism and sexism—she is strange, sexual, and grotesque because she is not white and not a man. It's terrible.
Lem may be drawing a parallel between science fiction and earlier exploration narratives. Fantasies of exploration often involve white Europeans going into remote areas of the globe and conquering or discovering non-white peoples, so Lem could be exposing the way that those stories and science fiction exploration are related.
Contact, whether with people on the globe or in space, involves an image in one's mind of the other as different. So when Kelvin says, "Rule them or be ruled by them: that was the only idea in our pathetic minds!" (11.10), he could be referring to science fiction space travel stories, but he could also be referring to imperial adventures on Earth. The message, then, is: Go out, find people who look different than you do, and kill them or enslave them. That's what exploration is all about.
So again, the book is probably using the black woman as a symbol of the way that science fiction stories of exploration are just another riff on older stories of exploration and conquest, which were often racist. Gibarian (whose visitor she is) is subconsciously revealing the subconscious of the science fiction genre, then, exposing the ugly fantasies that have often powered the genre.
It's also true, though, that Lem's explorers and scientists on the station are all white men. The only two visitors we meet, on the other hand, are both female. So you could argue that Lem himself sees women, and especially black women, as other/different/alien. If he's exposing the racist and sexist dreams behind science fiction, those dreams also seem, to some degree at least, to be his. Lem was, after all, a white man with a fondness for science.