The narrator of Flatland is the only two-dimensional citizen who has been exposed to the idea of a three-dimensional world. All the other residents of Flatland have lived the same life, experiencing the same things, for all their history, and so even the concept of "up" and "down" is alien to them. The narrator explains that every single thing in Flatland appears as a line, differentiated only by the size, distance, and effects of fog; because they can't see over themselves, they cannot tell what is a person, or other object without touching it.
As there is neither sun with us, nor any light of such a kind as to make shadows, we have none of the helps to the sight that you have in Spaceland. If our friend comes closer to us we see his line becomes larger; if he leaves us it becomes smaller: but still he looks like a straight line...
(Abbot, Flatland, geom.uiuc.edu)
In contrast, Spaceland (the three-dimensional world as humans understand it) contains height, length, and width, so an object can be rotated and examined from several angles. Flatland has only one angle because it has only one plane; Spaceland has three and so can be viewed on a larger scale. The narrator sees the contrast more sharply when he dreams of Lineland, where there is no dimension but a single line with line-shaped people moving along it; they cannot conceive of a second dimension, "side-to-side," but only "back and forth." The contrast between Spaceland and Flatland provides much of the discussion between the narrator and the Sphere.