A dream about keys is usually about access: to something shut, withheld, or already yours. The tradition of interpretation associated with Ibn Sirin tends to read the key as the means to an opening, a matter resolved, or an authority granted, and it holds that the reading depends on who is holding it. Artemidorus would agree on that last point and reason differently about the rest. Freud read keys as one of the dream's disguises and mostly declined to make them mean a single thing.
What the ta'bir tradition does with a key
The interpretation tradition known as ta'bir, attached to the name of Ibn Sirin, is careful and conditional. It treats a dream as a likeness to be weighed, not a message to be decoded, and it is explicit that the interpreter works by analogy and does not claim knowledge of what is hidden.
Within that tradition a key is a means of opening, and so it is generally read alongside what is opened. A key that turns easily, a door that gives, a lock that releases: these are read as ease in a matter that had been closed to the dreamer. A key that will not fit, or a door that stays shut with the key in hand, is read as an obstacle that has not yet moved.
The more distinctive move in this tradition is the question of who holds the key. A key given to you, a key you find, a key you have lost: the dreamer's position changes the reading. Where the key appears as something held over others, as access to a place that is not yours, the tradition is morally attentive rather than merely predictive. A dream that flatters the dreamer with power over what is closed is a dream to be suspicious of, not one to act on.
Where a symbol is disputed among interpreters, the tradition says so. It does not settle the matter by fiat. This is one of the reasons the reading is worth taking seriously as a tradition rather than as folklore: it is a discipline of restraint.
Artemidorus: the key cut to your own door
Artemidorus of Daldis, writing in Greek in the second century, compiled the Oneirocritica, the most complete dream manual to survive from antiquity. His method is empirical and situational, and it starts from a distinction he insisted on: between the oneiros, a dream bearing meaning about what is to come, and the enhypnion, a dream that merely reflects the dreamer's present state, like dreaming of food when hungry.
For a key, the circumstances decide. Artemidorus held that a symbol's meaning depends on the dreamer's trade, status, health and habits, so the same dream is a different dream for a householder, a merchant or a magistrate. A key that opens a storehouse means one thing to a person whose livelihood is inside it and something else entirely to someone with no storehouse. He would not have offered a reader a single answer, and he would have been irritated by the ones who did.
His reading is also backward-looking in a practical sense. The dream describes a situation the dreamer is already in, in the compressed language of image. It does not tell you what will happen next week.
Freud and the key as disguise
Freud's Die Traumdeutung appeared in 1900. Its machinery is by now well known: the manifest content of a dream conceals a latent content, and condensation, displacement and secondary revision do the hiding. A key, in that scheme, is a candidate for displacement. It stands in for something the dreamer would rather not look at directly, and the work of interpretation is to follow the substitution back.
Freud discussed Artemidorus in the book's opening survey of earlier writers on dreams, and he was clear that his own method was interpretive and retrospective rather than predictive. The dream does not forecast. It disguises something already present in the dreamer.
It is worth saying that his sexual reading of keys, which the popular caricature treats as the whole of the theory, was neither as mechanical nor as universal as it is made to sound, and that much of his account now looks wrong to researchers who study sleep and dreaming. The method still has readers. The mechanism does not.
Where the three traditions meet is on the question of ownership. All of them make the dreamer's own situation the place where the reading has to be done.
Where they disagree, and why that is useful
The ta'bir tradition reads the key morally and religiously, and holds its conclusions lightly. Artemidorus reads it circumstantially and empirically, and would treat a key as a fact about the dreamer's trade and standing. Freud reads it as a disguise, and would look for what the image is protecting the dreamer from noticing.
Three questions, then, to put to your own dream. What was the key for: a door, a box, a vehicle, a person? Who was holding it? And how did it feel in the hand, warm and familiar or cold and borrowed?
If the dream repeats
A single dream about a key is a single dream. If the same image returns night after night, and it leaves you unsettled rather than curious, the traditions are not the right address. Recurring nightmares and disturbed sleep are matters for a doctor or a psychologist, and that is said without hedging.
For one dream, the older manuals are enough. They were written by people who took the question seriously and who refused to pretend to certainty. That is the right company for a seven-in-the-morning search.
The key you actually dreamt of
Not a generic key. The one in your hand, at the lock you know. Whether it turned. Whether it was the right door. The tradition associated with Ibn Sirin would ask what the key opened in your life before you slept. Artemidorus would ask what you do for a living, and whether the door was yours. Freud would ask what you would prefer not to look at. Sit with whichever of the three questions gives you a flicker of recognition, and leave the rest.