Sigmund Freud defined repression as the way the brain defends itself against thoughts and feelings it finds objectionable by keeping those feeling, thoughts, and images away from the consciousness, as these things are incompatible with the individual’s ego. In his 1914 work, On the History of the Psycho-Analytic Movement" Freud argues that repression is "thecorner-stone on which the whole structure of psychoanalysis rests" (16). Repression is our brains’ primary defense in dealing with conflicts. It exists to protect the ego from our instincts. Therefore, repression is, in Freudian theory, the “universal” psychiatric process and exists in a separate part of the human psyche. Everyone represses some things: bad childhood memories or traumas, for example. But some people repress so much that repression becomes a major component of mental disorders, affecting a person’s day-to-day activities.

The concept of repression did not originate with Freud. The word had been used by the German psychoanalyst Johann F. Herbart but for Freud, it was not a concept but a fact. Studying his cases of hysteria, Freud determined that forgetting is something people actively do: it is intentional. When subjects were placed under hypnosis, they were about to return to these memories. After the memory was unearthed, often, the symptoms of hysteria would vanish. Freud, along with Josef Bruer, claimed in Studies in Hysteria (1895) that repression was a question of things which the patient wished to forget, and therefore intentionally repressed from his conscious thought and inhibited and suppressed."

Although the word “repression” has a life of its own in modern parlance, for Freud, it gradually became one of the four central concepts of psychoanalysis. First, Freud identified a “second consciousness.” This second consciousness is activated when the mind intentionally makes efforts to forget disagreeable events. The second consciousness isolates these upsetting memories from the normal course of thoughts. The psyche has effectively “disassociated” the unpleasant thoughts and “repressed” them, keeping those memories from being readily accessible. Until the full formation of this theory, repressed meant anything involved in the unconscious mind. By 1895, Freud argued that “repression” was the act of intentionally pushing away thoughts and images into the unconscious.

While the process of repression begins with the intentional pushing away of troubling memories or images, once those thoughts have been sent to the “second consciousness,” control is taken away from the individual. (This “splitting” is more fully explored by French psychonalysts Jean Laplanche and Jean-Bertrand Pontalis in their 1967 work The Language of Psycho-Analysis). When the memory moves to the second consciousness, this is when repression occurs. Freud called this the "nucleus and centre of crystallization" in Studies on Hysteria. The ego has determined that disagreeable thoughts are not allowed to enter, as it were. It treats these thoughts as “non-arrivals” and works to repress them. However, even when the second consciousness takes over, it is not always effective in completely freeing the subject of the pain of negative experiences; frequently, when instincts are suppressed, the affect can be that the individual experiences anxiety, a type of bridging of the second consciousness and the conscious.

.Repression is very common. Everyone experiences repression from time to time, although the severity of the events the mind tries to suppress can end up resulting in mental disturbances and/or illness. Some of the negative effects of repression include hysteria, obsessive compulsive disorders, and hallucinations. Freud discusses these illnesses in The Neuro-Psychoses of Defence (1894).

Freud’s work in trying to understand repression continued and by 1915, he was able to distinguish three phases of the process: fixtion, repression proper, and the return of the repression. He describes several stages in the process. First, “repression and the unconscious are correlated,” meaning, he says, that there is sound reason for accepting the argument that there is a basis for “primal repression.” In this stage, an “initial unconscious nucleus” kicks in, deciding what will be repressed. This happens through fixation in the unconscious.

Following fixation is the second stage: repression proper. In German, the phrase is “eigentliche Verdrängung” which translates as “after pressure.” Here, the material that needs to be repressed goes into action mode. It takes those initial, fixated thoughts and actively moves them into the second conscious.” This is not easy transference, however. Freud likened the second phase of repression to the manner in which tourists are conducted to the top of the Great Pyramid of Giza by being pushed from one direction and pulled from the other" (from a note added in 1915 to his 1905 study Three Essays on the Theory of Sexuality).

The last stage is when repression is not completely successful, and those unpleasant thoughts, feelings, and images resurface in another form. This is “return of the repressed” and it may come back to the brain in dreams, slips of the tongue, or by misplacement of objects.

Source: International Dictionary of Psychoanalysis, ©2005 Gale Cengage. All Rights Reserved.

In psychoanalysis, repression refers to burying a bad  experience inside rather than  bringing it out and discussing how to handle the bad experience.  For example, I have someone close to me who was raped by a cousin, but instead of dealing with the experience with a therapist, she has repressed the memory of that whole bad scene and pays for it today with fear, headaches, and depression.  A soldier who comes home from war with horrible memories of what they have seen but doesn't see anyone about how to deal with the recurring scenes in their minds, has repressed or buried inside themselves those memories.  The problem with repressing bad experiences rather than dealing with them is that the repression only delays dealing with them.  In the meantime, people do crazy things to relieve the pain such as take drugs, commit suicide,or live  with  depression which is anger turned inwards.  Some of the tragedies we see in the news are the result of repression of terrible experiences which need to have been dealt with but never were.