There's a lot in Solaris (at least in the 1976 movie and the book). In almost all sci-fi which contains aliens, they're easily comprehend-able by humans - they have motivations we can describe in our own language, a physical form that while strange is likely a mashup of earth creature characteristics (after all that's where our vocabulary comes from). Solaris asks what it might be like if this isn't the case, and where the limits of our comprehension are.