Surrealism in cinema refers to a movement in Paris in the 1920s that eschews the traditional, formal patterns of film for jarring juxtapositions and disjointed narratives. Surrealist films are just a part of an entire surrealist movement that involved many different forms of visual art that sought to imitate a dream-like state with association taking priority over logic.
Surrealist filmmakers actively fought against causality and rationality; trying, rather, to appeal to a deeper, visceral response from the audience. Apart from that, surrealism is a mish-mash of styles and forms, constantly playing with continuous and discontinuous editing, visual imagery, and fragmentation of time to create an ultimately disorienting experience.