A door is the smallest piece of architecture with a whole symbolism attached, because it is not a place but a passage. The comparative study of ritual has made the threshold a technical term: rites of passage, in the account descending from Arnold van Gennep, turn on crossing from one state to another through a marked between, and doorways have collected guardians, blessings and observances in most cultures, from household gods at Roman doorposts to the mezuzah on a Jewish doorframe. Dream interpreters inherit all of that when a door appears.
The psychological traditions read the door by its state. Open, it is opportunity, invitation or the next necessary step; closed, it divides into the locked door, which is refusal or a secret, and the merely shut one, which asks only to be tried. Doors that multiply down a corridor, doors that open onto nothing and the door the dreamer cannot find again are recurring reported forms, each pulling the reading its own way. Freud's school added its characteristic bodily reading of openings, which belongs to that theory.
The folk dictionaries read doors toward callers, offers and news. Across every register the constant is the same: the door means nothing in itself, and everything by what stands on the other side and whether the dreamer turns the handle.