Solaris is an apparently self-organizing body covering the entire surface of a planet orbiting a binary star system. It came to the attention of space-faring humanity because the orbit of the planet is anomalously stable, a fact that is ultimately attributed to the purpose and capability of Solaris. Humanity has established a local research station with the mission of studying and making contact with Solaris. The "ocean" (as they have dubbed the vast, colloidal mass) is hardly placid but rather practiced in creating objects and structures of varying permanency and in occasional defiance of laws of human-observed physics.
Decades of futility and occasional, accidental deaths have reduced the staff of the research station to three, including the former mentor of our protagonist. Shortly before we arrive at the station, the scientists have decided to conduct an unprecedentedly provocative experiment involving the exposure of the ocean to powerfully directed x-rays. Correlation is never here established to be causation, but soon afterward the scientists are visited by "Guests," people of their acquaintance who (for a panoply of reasons) cannot possibly be on the station and who have no memory of how they arrived. This is now a common trope, but Lem gets some clever tension out of it, primarily because the surviving scientists don't see fit to warn the protagonist when he arrives at the station (a less focused writer would have spent more time with the scientists mistaking the protagonist for another "Guest").
What distinguishes Solaris from other aliens-in-grandma's-clothing is that there seems to be no purpose to the "contact" or even any true awareness of the humans. The analogy I find most apt (and which I haven't seen in other discussions of Solaris) is that of an allergic reaction by Solaris to the humans in its noosphere. The Guests are formed from human memories much as antibodies learn the shapes of foreign bodies.
There is a wealth of implications here. I'm fond of the existential paradox (?) of humans seeking extra-terrestrial intelligence and confronting only themselves. Also gratifying are the conjectural and wry responses by the scientists; very little Lovecraftian Sturm und Drang in the face of galactic absurdity. The responses by one of the Guests when she grasps the nature of her origin are refreshingly inconclusive. How much of another person can we preserve in our memories? How much do we need?
A more mischievous writer might have created a cult of Solaris, galactic pilgrims who believe the ineffable ocean to have been responsible for our millennia of religious phenomena. Lem raises this idea and disposes of it as almost a mathematical digression.
It's difficult to compare physical and temporal scales between humanity and Solaris, so the conclusion of the novel is ambiguous as to whether Solaris has resisted or succumbed to the infection by humanity. Our infection by humanity, of course, remains terminal.