Liminal space is kind of a fuzzy concept. Liminality is the quality of being transitional, between one status and an other, transitory. The blob of living tissue in a cocoon is in a liminal state, neither caterpillar nor butterfly. I feel safe in assuming you know what spaces are, so when we put them together we get spaces that exist to link other spaces or that punctuate time between other spaces. The most obvious examples of these are hallways and waiting rooms.

That covers the academic or technical definitions but as with so many terms its usage has evolve beyond its obvious meaning. Liminal space typically refers to a tone or aesthetic where the place portrayed gives off a mildly lonely, creepy, or eerie vibe. They feel slightly off putting despite not actually containing anything objectionable. They are liminal not in a spatiotemporal sense but on a deeper level where they do not belong in the real world but don't quite cross the line into surrealism. While it's hard to pin down too many elements one consistent feature of liminal spaces is the lack of people; either in a space that is inherently antithetical to human congregation (think maintenance tunnels and unfinished attics) or that would normally have people in them and who's absence is unsettling (playgrounds at night or empty ball rooms). Other features associated with them are overly long or narrow hallways, under or unfurnished rooms, dim and uneven lighting, and questionable or confusing architectural design elements. Beyond that it's in the know it when you see it category. Liminal spaces as a concept seemed to have been birthed in conjunction with the Backrooms which is simultaneously the prototypical example and too eldritch to be included without an asterisk.