Raymond Carver's short story "Cathedral" is about a husband and wife who are visited by an old friend of the wife's, a blind man named Robert. The story is narrated by the husband, who remains unnamed through the story but is referred to by Robert as "Bub."
(1) From context clues and dialogue, the reader can deduce that the unnamed narrator is a middle-class white man. He works a job he doesn't much like, drinks and smokes and watches a lot of TV, and seems to be not too happy in general. The narrator is bothered by the blind man visiting because he has never met a blind person before and doesn't know how it will go. He says, "My idea of blindness came from the movies. In the movies, the blind moved slowly and never laughed." He seems to conclude from this limited knowledge that it will be uncomfortable and possibly boring to have a blind guest. In addition, his wife has a pretty close relationship with the blind man—they have kept in touch over the years by recording tapes in lieu of written letters—and he seems to harbor some confusion about their closeness, if not jealousy.
(2) When Robert asks the narrator to describe a cathedral, the narrator seems stumped by the question, mostly because he has never been asked to do anything like that before. He attempts to describe a cathedral mostly in loose terms: "very tall," "so big," "massive," and "built of stone." He repeatedly says that he isn't doing a good job, but Robert never confirms this. His struggle to describe something to someone with no visual context makes him self-conscious and reveals, perhaps, the intellectual and imaginative limits to which he has been pushed in his life.
(3) When the narrator and Robert are drawing the cathedral together, Robert tells the narrator at one point to close his eyes and continue the drawing. In closing his eyes, the narrator might begin to be able to imagine the way the drawing (his interpretation of a cathedral) might feel to Robert. In the exercise of drawing, and of closing his eyes, the narrator might be getting a better sense of how to describe something visually/spatially, which he could not do before the exercise. As for the other parts of this question, I hesitate to give some moral/symbolic significance to Robert's blindness (even if it was Carver's intention). Disability is so embedded in the symbolism of literature—the effect of this is that we are never able to read (or interpret) a disabled character through the lens of lived experience rather than some symbolic resonance. Disabled characters (especially blind characters) are often used (or interpreted) as "foils" to seeing characters, in order to reveal the irony of how little the seeing character can really "see" the world. That is one way to interpret this story, but it is also limited.
(4) Robert points out that generations would work on building one cathedral and that one person would often not live to see the completion of their life's work. He says that "they're no different from the rest of us" because we are all works in progress and are never done learning, or working, or improving. We all have things we will not see the end of. What the cathedral means to the narrator is harder to pin down—Carver leaves the ending of the story fairly ambiguous. Robert tells the narrator to open his eyes and look at what he's drawn, but he keeps them closed, sits in the not-seeing, and the story ends with him saying, "It's really something." That he can say the cathedral is really something without looking at it says more about how he feels about the process of drawing it and the experience he shared with Robert. In some way, the cathedral symbolizes "process" for both men. It is less about the finished cathedral (real or drawn) and more about the significance of making it.