Freud believed that the mind could be divided up in a spatial manner. He chose to call those sectors: conscious, preconscious and unconscious. According to Freud, the preconscious is located in between the unconscious and the conscious locales. Think of it as a membrane through which a piece of repressed memory passes.
Freud described the unconscious section as a big room that was extremely full with thoughts milling around while the conscious was more like a reception area (small room) with fewer thoughts. The preconscious functioned as the guard between the two spaces and would let only some thoughts pass into the conscious area of thoughts. The ideas that are stuck in the unconscious are called “repressed” and are therefore unable to be “seen” by any conscious level. The preconscious allows this transition from repression to conscious thought to happen.
Example: In general, people are not thinking of their address or telephone number in a given moment, but if you ask them for the information they can easily recall and provide it.
Fournier, G. (2016). Preconscious. Psych Central. Retrieved on October 16, 2017, from https://psychcentral.com/encyclopedia/preconscious/